Ubuntu Feisty now supports Zeroconf networks. All you need to do is collect a few Ubuntu computers together, hook them up to an Ethernet hub and watch them talk to one another. No fuss, just a simple ad-hoc network in seconds.
Press System -> Administration -> Network, select a network connection and press Properties. Then, under Connection settings, change the Configuration to Local Zeroconf network (IPv4 LL) to connect to a Zeroconf network.
While Zeroconf looks great in principle, I've never been in a situation where it would make sense to use it. I suppose that if you regularly go to conferences and meetings it saves having to take a load of equipment with you (and the mandatory half an hour asking questions like 'How come I can ping you but you can't ping me?'), but apart from that, where else is such a network set-up useful? Answers on a postcard please.