The Ultimate Cheat - TestingCheatsEnabled exposed
The advent of Sims 2 adds a cheat, primarily designed for the testing team, that allows players to trigger all kinds of events and skip all the usual mundane stuff that the rest of us have to slog through. This cheat opens up whole new avenues of gameplay and yes, it's even more powerful than the move_objects on cheat.
The cheat is: boolprop testingCheatsEnabled true. Enter this cheat into the game the same way you enter any other cheat: Hold down CTRL+SHIFT+C and type it into the white box top left. To turn it off, replace "true" with "false".

Entering the cheat in the Sims 2
While move_objects is a nice cheat (and has existed ever since Sims first came out), all it really does lets you do is select and move things that you ordinarily can't touch. If you want the mailbox in the back garden, that cheat will do it - type it in, grab the mailbox and shift it.
The testingCheatsEnabled cheat offers one hell of a lot more options to play with. With this cheat you can kill anybody in any way you like (kids can die from old age for example), you can give a sim a bladder problem by fiddling with the appropriate need to keep it down, or give everybody a boost with one mouse-click. Alternatively have papers delivered in the evening, or summon a burglar on demand (if that takes your fancy). Most objects have hidden options - have a play.
Note that this is a very powerful cheat - you will probably lock up your game while using it so please use a non critical Sims family for testing this cheat. Neither I nor thesimszone.co.uk will accept responsibility for breaking a loved family with this cheat, nor for corruption of neighbourhoods (sim or otherwise). Oh and don't leave your chewing gum on the bedpost all the night either.
The Mailbox
Let's look at some of the things this cheat opens avenues to.

Page 1 of the new mailbox options
Activate the cheat, choose a sim and click on the mailbox. You'll see this. Here are shortcuts to useful features. Here you can force bills, papers or burglars to arrive, or you can teleport a sim regardless of how good they are at logic.
One of the most useful options is the Make options. Make me know everybody, make all happy and make friends for me do what they say on the tin. Invite all Neighbors needs a bit of kid glove handling, as having hundreds of sims on the lot will probably kill the speed of the game.
Elsewhere on this screen, have a look at the Motives. The "Make All Motives Static" will freeze the decreasing of all eight needs, which means once you top them up either in the game or through the following cheat, they won't ever turn red (unless you fiddle with them as shown below). Its basically mediating.
The other option, Make All Motives Dynamic, is the old fashioned "point and click" way of altering a sim's needs. Select this, the needs will drop as they do normally throughout the game. However if you was to left click on a needs bar, you'll be able to change its value up and down. This includes the Environment need, but it resets itself on demand. The cheat also works on the relationship scores and skill points.
Therefore if you want to make a sim wet himself on demand to spread the memory around, this is one way to do it. Be aware of other triggers in the game that the needs are related to - empty social and/or hunger scores in babies/toddlers/children for example will bring the social worker. Empty energy will obviously force a sim to seek out a bed, low fun will drag the rest of the mood down and stinky sims will, well, stink basically.

Page 2 of the new mailbox options
The second screen offers you the chance to trigger somebody's birthday, display the value of the lot or force a walkby/visitor. Alternatively if you want to be a miserable goat, disallow all visitors.

Page 3 of the new mailbox options
Up To Mischief...

Break as you go - now possible
Holding down left-shift and left-clicking on an item brings up new menu choices to play with. Something breakable such as the toilets or showers will have options to break or clog accordingly. A real excuse to get the handyman/woman over and into bed, that. Other items have new options that appear wheny ou click on them in this way but most have Force Error on them which doesn't do anything exciting.
Depending on what you click, you'll be able to age the appliance, make it as broken, or clog it up. The fridge has a useful Replenish option, avoid all the hassle with the home delivery.
Simology

Customise your sim like never before with the hidden menus Page 1
Hold down left-shift and left-click on a Sim opens up a whole host of new options, such as making them fat/thin, altering their aspiration score and making them grow up at 7pm that same day, to avoid all that growing up malarky.
From this screen, you can change the aspiration score to any one of the five segments, to cheer a sim up or depress him. Note it isn't possible to change the aspiration level in the way it is to change the needs - it has to be done this way. You can change suits but they rarely match up with their labels. Oh and Sims won't stay naked either - they're modest like that.
Top Tip Time: Babies can be made selectable with this method of cheating, allowing one to see their needs (and a naff looking photo but not a lot else). To do this, shift-click on baby, choose to make him unselectable (he'll disappear from the face list on the left), then click him again to make him selectable and you can see his needs. You'll find now that baby has a full set of needs, no wants, no fears and his relationship score towards the parent.

Customise your sim like never before with the hidden menus Page 2
The second page of options is very interesting. Here you can change any sims aspiration to any other one in the game, so if you gave Bob a Knowledge aspiration and fancy sending him down the Romantic aspiration instead, here's how you can change it. Kids can have one of the adult aspirations as well in this fashion.
Take note of the Spawn menu. What's in it you may ask? They are extra tools and things named after various members of the production team that trigger things in the game the easy way.
Spawn of Satan

The devil makes work for idle hands - and the grim reaper
Now this is the fun stuff. This is where you bring up the various bits and pieces that trigger the various parts of the game, be it death, illness, chance cards or the clothes tester.
Selecting an option spawns either a tombstone or a big cardboard box. Choosing to use something that just appeared will usually happen immediately as opposed to joining the end of the queue.

(L to R) The Chance card tester, Reaction Tester, Scenario Tester, Clothing Tester, Tombstone of L & D and Rodney's Death Creator
The Chance card tester shows all off the cards in a career but doesn't really benefit anybody.
The Reaction Tester and the Scenario Tester have no real use for most people so ignore them.
MakeMeSick will allow you to, um, well make a sim sick really.

Clothes Tester in operation
The Clothing Tester will force a sim to cycle through his entire collection of clothes as well as change them. Note that this will throw an error message up on children about materity outfits and also an error about the work-out outfit for children. For adults you can dress in any style from any level of any job, or dress like an NPC of your choice.

Tombstone of L &D
The Tombstone of Life & Death allows you to add new members to your family with just one click. This will not make them a part of your family initially - you'll have to do that manually by clicking on the tombstone of L & D.
You'll see from the screengrab that you can add any age of either gender. They'll just suddenly appear out of thin air. Note that taking children in this way just generates a random child without taking genetics or anything into account so this is basically adoption without a social worker.

The Death Creator
Here we are, nine ways to die in Sims 2. Officially, it's impossible to kill kids in most of these ways here (adults can die from any of them but old age). Until now. Select the sim you want and pick how you want them to die. Death by Fire looks wierd on its own as the animation was designed around flames and not to be done on is own. Yes, this works for kids as well who can die from "old age" at the grand old age of, um, 12.

Dead Simkid with life insurance
This was a kid, 12 years old, who died from "Old Age". Fascinatingly enough, the insurance company still paid out. Such was the shock of a kid dying from Old Age, it made this sim streak across the lot to bawl his eyes out at the death of his younger brother.
Dusty Bin

New dustbin options
The dustbin sprouts new options as well, such as checking the job status and salvaging items, although most of these options are irrelevant to end users.

Oops!
You'll see this message a lot if you force errors. They basically throw debugging information to a file, something which is no use to anybody outside of Maxis. Always choose Reset, not delete as you'll lose said item.

Another one bites the dust
Something that can't happen in the game - sim boy squashed by satellite falling out of the sky. I wonder how that happened? ;)
In summary, a nice little cheat that. Now one can concoct a whole range of new sequences by intimately playing with the parameters to set their sims up as they want them, not as they are at the time. Have fun :)