For quick boot times you can’t beat Ubuntu – I have it dual booting on my laptop. Currently I’m “playing” with 11.04. It’s going well, apart from umpteen program faults – but it keeps on rocking along even after those! One thing that I found the other day which I thought was REALLY useful is an app which is in the Software Centre called Minitube. 50.4 Mb of download later, and a quick installation you’re now able to watch YouTube videos without Flash getting in the way.
Update: There is a version of Windows too (costs 9 Euros though) : http://flavio.tordini.org/minitube
