Describing itself as "napster for C programmers", this handy little tool from Martin Pool allows you to use the CPU power of all the compatible hosts on your local network. Unlike some similar tools, it doesn't require them all to have the same filesystems or configuration.
distcc works by running a small daemon called distccd that listens on port 4200 of each host. You probably want to be careful with that because it allows anyone with a client to run any command. I strongly suggest that you run it as a totally unprivileged user that owns no files (eg make a new normal user called "distcc"), only run it when you're actually going to use it, restrict it to hosts that you control with iptables and so on. Generally, be a bit paranoid about it.
The reason to use it is performance. On small compiles, it only shows itself marginally. Using distcc to build itself across two hosts here shows an improvement from 42.24 seconds to 24.78 seconds. Of course, there's normally a lot more than a straight compile to building a package. Building a simple debian package of distcc, including running configure and making the archive, shows only a slight improvement from 50.54 seconds to 45.37 seconds[1]. Still, it'll be worth having on larger projects.
Apart from security, the other drawbacks (besides security) are that the network usage can go through the roof for large compiles (so, as for security, you probably don't want to do this on public systems on a public network) and that usage is currently restricted to gcc. It is a fairly young program, though, so who knows where this might go?
Getting started with distcc is fairly easy. You just download the code from http://distcc.samba.org/ and compile it. It's not a very big compile, which is fortunate, because you can't use it to compile itself before you've compiled it for the first time... after that, you might find yourself wanting to use it for every compile!
[1] -- No, that doesn't make sense. I don't understand either.
OK, the above is uploaded as http://www.alug.org.uk/contrib/?DistCc under the TipsTripsAndWorkarounds category, so feel free to improve it. If you want to ask me more questions, post to the list.