We've been playing Unreal here at work for some time and now people are getting bored with it. So my question for you all is... Whats a good cross platform network game? We have three mac's and 4 pc's to work with and we need a good game that we can all play.

Is this the original Unreal? If so, I'd suggest Unreal Tournament or Quake III for first-person shooters. Both have enough mods out there to make them fun for a long time. One of my most favorite games to play on a network, though, is Myth II. It's a great real-time strategy game. Plus, I believe the Myth Total Codex (which has Myth, Myth II, and a ton of maps) is only $20. There's just something about dwarves with moltav cocktails...
Try some of these:

Quake 3 Team Arena
Elite Force
Total Annihilation

Need more? Just ask!
We've been playing unreal tournament:) Could you guys also post the website address with game titles please (if you know them) Thanks. Also a racing game would be awesome!

I don't know these websites...sorry. But here's some racing games that I like:

Nascar 2001
4x4 Evolution
Madden Racecar 2000
Box Derby

Again: sorry, I just can't remember the websites.