To prepare a joint-presentation or other workgroup-stuff, a shared (auto-sync) folder without the need of a central server is a nice thing to have. A few years of Groove definitely spoilt me - but there will be no Mac version in the near future (remember: Groove-founder Ray Ozzie, see also here, way back also foundet Lotus and is now chief software architect for Microsoft) .
Now I found FolderShare - a free service by a small company called ByteTaxi (which was aquired by Microsoft in 2005). They provide clients for both Windows and MacOS, no fancy features but pure p2p file sharing, encrypted, closed workgroups, fast, no-hassle. It's operated mostly in a browser (I tried Firefox and IE under Windows and OmniWeb and Firefox on Mac - no problems).
Groove still is way cooler - but Windows-only :-( Foldershare does what it's supposed to do - seems to be "here-to-stay". At least for me and my (small) workgroup :-)