FarSight is a MacOS utility that enables editing of the textures and other resources used in The Sims.
Most of the data in the game is stored in file archives (FARs). Within those archives, much of the data is easily recognizable -- for example, all the textures are stored as BMP (bitmap) files.
FarSight lets you browse the contents of a FAR archive, and extract any item into its own separate file. It will even preview most BMP files for you (but it will extract any file at all). You can then use your favorite editing tools (I recommend GraphicConverter) to edit these files to your heart's content, or replace them completely.
Then, here's the neat bit. The clever folks at Maxis designed their game such that, if a file with the name they're looking for exists in the same folder as the FAR file, it will use that file rather than the one in the FAR. So, to override a particular texture, just extract it into the same folder as the FAR it came from, edit it, and leave it there. If you later get sick of it, just delete that file, and the game will resume using the one in the FAR archive.
FarSight ( OS X)
FarSight (Mac OS 8-9)