SheepShaver is an open source PowerPC Macintosh emulator for use on both PowerPC and Intel Macs (and other systems, but that's outside the scope of this post). It enables the running of some types of classic Mac software which cannot be run any other way. It doesn't support games, and probably never will, but that doesn't mean it's of no use to Mac gamers. More on that below...
SheepShaver is not a commercial product and is in development, and as such requires some dedication by the end user to install and run. It requires a classic Mac OS install disc (not tied to a particular Mac model), and a Mac ROM image. It's beyond the scope of this post to go into detail - for more information on the requirements, installation, and running of SheepShaver, follow the links below:
SheepShaver Wikipedia page
SheepShaver article at Mac OS X Hints
SheepShaver article at Low End Mac
Although SheepShaver isn't a gaming solution, it does have some relevance to those people who are still using the Mac TRLE (Tomb Raider Level Editor). The TRLE includes a carbon Level Player that should run natively in Leopard on a PPC Mac, and in Rosetta on an Intel Mac. This is fine for those who are downloading already compiled, ready-to-play levels in TR4 format, but they might come unstuck with levels that are downloaded as WADs (the TOM file, along with other WADs), and which need to be compiled using the 'Tom2PC' converter included with the Mac TRLE - this converter is a classic Mac programme that can't be run in Leopard on any PPC Mac, or on any Intel Mac at all.
This is where SheepShaver may be useful for Mac TR players. If you have no other alternative but are eager to be able to convert and play a level posted as WADs, then SheepShaver might be able to run the 'Tom2PC' converter, thus giving you a compiled TR4 data file to run in the carbon Level Player!
As usual, please note that I don't have access to an Intel Mac, or a Leopard PPC Mac, so I'm not able to do any testing to verify any of the above. But in theory it should work, provided you're able to get a functional install of SheepShaver on your Mac.
Thanks to 'JQ' for bringing the 'Tom2PC' issue to my notice, and to Inside Mac Games for the referral to the Low End Mac article!