I'm surprised no-one else has posted this, so I thought I might as well!
Ubuntu is a free operating system, and is a great alternative to Windows / OSX. I've been using it for just under a year, and I've had no real problems whatsoever. The latest version is the best yet; it's got out-of-the-box support for lots of mobile broadband devices (great for netbooks!) and even more webcams are now supported.
On my machine, it seems to boot about 15 seconds faster, and is a great option for old and lower-spec machines. It's works fine on a machine with 256MB of RAM, and there's always it's lighter-weight cousin Xubuntu for even older machines.
The download comes with everything you need to get started; a browser, a full office suite, media players, games, photo-editor, instant messenger ....
It's also available in a huge range of languages and has excellent assistive technology for impaired-sight or hard-of-hearing users.
And, most importantly for HUKD: it's 100% free!
It's free to download, or you can request a free CD here: https://shipit.ubuntu.com/
P.S. Let me know if you think I've missed anything important out in the summary!
EDIT: Just to clarify - this is a new release of Ubuntu. The previous release came out in April. It got very hot - 500+. However, like this one, it was also voted cold to begin with. Don't vote this cold people: it might save you £100 or so instead of shelling out for Vista, and Windows 7 when it comes around.