Ares Galaxy is a quite popular open source P2P client for Windows, maybe as good as Frostwire. In particular, the Ares network has different users than Gnutella so you can find many other files there.
I used to run Ares in Linux installing first WineHQ, the most popular and user-friendly way to run many Windows applications in UNIX-based systems, not having Windows at all. Ares works in a very decent way with WineHQ, so you can just install the latest version of WineHQ for Mac et voilà.
Another option is using a commercial WineHQ-based software called Crossover. Ares Galaxy is not in the supported applications but it have worked perfectly for me so far.