All of them :) Well, almost. Minimum requirements are a platform ...

• with at least a 32bit processor,
• with a reliable power supply (forget those handhelds, crunching takes power),
• with 750K of real (not VM) memory (as of Jan/2000),
• with some means of transferring work to/from a keyserver,
• supported by a decent c++ compiler
• with more than a handful of installations
• with someone to do the dirty work

The list of currently supported clients is on the the official client download page.