Basically the title says it all. Is it possible to play a multiplayer game of Warcraft 3 between a Mac computer and a Windows computer? Thanks for the help.
Yes. I've done that often. There are no issues at all, aside from the standard "do I have the right ports open on my router and firewall" ones common to any such gaming.
Luckily War III traverses NAT so there usually isn't any issues connecting to battle.net. Starcraft you still needed to open ports. If there's one thing blizzard is great at it's their cross platform support.
Even in the pre-OS X days, when my Macs and my friends' PCs couldn't see each other at all on the network, Starcraft and Warcraft 3 always let me see and join the games hosted by the Windows machines.