LimeWire is the best Gnutella client I have tried so far (but that's only my opinion). As Marvinius wrote, it's entirely written in Java and should therefore run on any platform where a JRE and a network connection are available. No, make that a JRE, a network connection and half a gig of RAM to keep Java happy.
Interesting features:
  • Multiple simultaneous searches
  • A cool spinning lime slice when a search is being processed
  • Ability to throttle upload bandwidth
  • Ability to filter out crappy hits, like "x" and "x.mp3"
  • Hasn't crashed on me yet

Update as of 2002-06-18: As everyone should know by now, the Gnutella network genuinely sucks. I don't use it anymore.