There is Linux, of course. I have been using Ubuntu as my main desktop OS for several months now and like its speed and simplicity. It is quick to boot up, the layout is clear and, coupled with a Nvidia video card, provides really good graphics.
There is a plethora of other distributions of Linux (including RedHat, Fedora, openSUSE, Sabayon, Mandriva, etc). But there are a couple of other not so well known gems.
I tried Linux Mint 7, booting it from the disc provided free with Linux Format magazine. Very quick, very green, delightful. Since my Sony Vaio does not take kindly to partitions and dual booting, running off the disc was the only alternative for me - but it worked well, including hitching up to another WiFi system when Windows refused to do so.
The advent of netbooks has meant that these light laptops need light OSs. Reading Doug Belshaw's analysis of the ideal OS for netbooks made me want to try gOS ("good OS").
Here he is describing its features: