This bit was added after I read on a Mac Simmer's page that any download ending in ".zip" or ".exe" simply could not be used on the Mac. This is not true. Modern unStuffers will also unpack zip files and even "exe" files that are in fact self-extracting zipfiles. The real problem is that Mac users can't get on the Maxis site to download these self-extracting zipfiles, because to even get an account and log in they need a serial number of the kind found on the PC game's CD case or manual. And that's sad.
Concerning zipfiles and exe-files that can be downloaded, however: as long as they can be uncompressed into the original files, they can be used. They simply need to be put in the right directory (see What goes where). And, of course, they need to be compatible with that game version. But the same applies when using them on a PC.
One small problem Macs have that PCs don't: the 31-character limit. Objects with longer names won't be loaded in the game. One solution is to put them in a (short-named) FAR file, but apparently Mac FAR makers can't be trusted. (This was the situation in 2004, but a FAR maker has since been written by the maker of IFFSnooper, and it is probably reliable.) A much better solution is to rename them with Namer, a very barebones utility that needs Classic to work, and puts the long name in the file's comment while calculating a shorter name.
The filename length is usually only a problem with skin files, especially mesh names. A utility that shortens these names is SkinShrinker, but I very much prefer Namer, which preserves the original name rather than just chopping it off after so many characters, risking filename conflicts (when two files ending on "_dressgreen" and "_dressred" suddenly both end on "_dres") and the breaking of dress sets.
(Sadly, now that Aspyr has moved on to The Sims 2, its old Sim downloads, including Namer, seem to have become inaccessible.)
To conclude: any Sims file that can be used on the PC
can also be used on the Mac. You just have to know what to do with it.