Sooner or later when you play the Sims 1, you’re going to start wishing you had a handy tool to make playing the game easier, create your own custom content and fix problems you may encounter. If you’ve reached that point, here is a list of programs that you’re sure to find helpful.

SIMS TRANSMOGRIFIER
The Sims Transmogrifier-- or T-Mog, for short-- is the program to make your own objects. (Yes, folks, this is the one you use to make it happen!) It's very easy once you get the hang of it; however, in the beginning it takes some getting used to. The original Transmogrifier site seems to be broken. However, you can download a copy of T-Mog below.
FILE COP
Downloading can be fun, but sometimes it comes at a price.Whenever your game acts up after you've installed new skins, chances are it is because of a bad skin file you just downloaded. This program allows you to check and remove whatever bad skin files are causing your game to shut down.
SIMPOSE
Simpose enables you to take your Sims and pose them against a background of your choice. You can also use it to add new animations to the game. Many skinners and hackers used this program to show off their creations or create new animation. Some creative people used it to create cool graphics and comic strips. Definitely get this if you think you'll be skinning in the future or want to make graphics and comic books using sim characters.
IFF PENCIL 2
This is a very handy program with a lot of uses, such as hacking, viewing iff files, and curing that annoying Upside Down Head Syndrome that everyone likes so much ;-).
HOME CRAFTER
With Home Crafter, you can create your own floors and wallpapers by importing graphic files. It's quite handy, especially if you're good with a paint program like Photoshop.
ELI SIMS
If you don't like swapping families between neighborhoods the hard way (by copying .FAM files from the export folder into the import folder, etc.), you can use this program. With a click of the button, you can import families and houses from one neighborhood and bring them into another. You can also view an image thumbnail of .iff files-- a very handy if you like to build houses for download!
MAKE A DATE
This is a great program if you can't stand the Townies your game generates. Simply create your own Townie, then replace a pre-existing one with it.
SIM METAMORPHOSER
Available at Paladin's Place (SimWardrobe.com), this program allows you to change the head and name of any one of your Sims. It's popularly known as the program to cure "UCS"-- or "Ugly Child Syndrome."
FAR OUT
Also available at Paladin's Place is Far Out, a program in which you can view .far files and extract .bmf to .skn files. This program is invaluable if you want to use Sim Pose, because it allows you to import all the .skn and bitmap files from The Sims into it.
SIMS FILE MAID
I personally haven't used this program myself, but apparently it makes installing downloads easy for those who don't like to deal with WinZip. Works with all object files, with the exception of house files.