Synergy: a configuration text file to edit, and you can control all your computers from the same keyboard and mouse. It’s completely free and open source, runs on Windows, OS X and Unix, and, more importantly, it just-works, immediately, no trouble at all, in the background, not even preventing you from using each computer’s keyboard and mouse when you need to.

Except, of course, it’s not so pleasant having to control a Windows system with a Mac keyboard (and vice versa, presumably), and I’d rather the control could be reciprocal rather than having to define a client and server, but it’s still impressive and useful.

Probably not advisable for university dorms and some cable networks, however, since there isn’t even the most basic password system.