I use the IPEVO Free-1. I've bought one for myself and several for friends. All of them work great. I chose it because it comes with mac drivers (although, annoyingly, the "driver" is a program that stays permanently in the dock). It also has Windows XP drivers, which seems to suit your requirements.
Good phone - it's smaller than you think when you look at the photos. Quality is great, very little to no echo whatsoever.. integrates nicely with Skype.. fits on your desk. All they need is a bluetooth version, because the wire is a tad annoying.
Just my $.02. Not only the phone itself works nice with OS X, the company's customer service is good too. My unit failed about a month after purchase, but was promptly replaced with a new one.