Things to do with an old computer. Anyone can add more ideas?

Let's say you've got an old Pentium 4 machine that you want to make use of - nothing ancient, still perfectly capable of doing useful work. If you use it as BitTorrent machine, it'll easily suck up 150w. As a distributed computing node, more like 300w. Running 24/7 that's 1314 kW/h or 2628 kW/h a year.

In the UK, cheap electricity costs around 13 pence per kilowatt hour, meaning that this 'free' old machine will cost you £170 a year with a modest workload or £340 a year if working hard. An Atom-based nettop will consume around 30 watts under load, will have comparable performance to a fast Pentium 4[1] and can be bought for less than £200.

The only reasonable thing to do with an old computer is discard it, or recycle it if you don't mind the idea of a Chinese child stripping it for parts[2].

[1] http://www.cpubenchmark.net/cpu_lookup.php?cpu=Intel+Atom+N2... [2] http://news.bbc.co.uk/hi/english/static/in_depth/world/2002/...