If there is one thing only that has to be cluttered, then make it all your passwords. You might not have heard of LulzSec and if you haven't it's no biggie, since the important part is what they have in their possession now (they and many, many others on-line), namely a long list of email addresses and their passwords. Most passwords are ridiculously easy for people with suitable software to crack within a few hours and so I'm urging you all to revise your chosen words.
How about 11111? Or aaaaa? abc123 anyone? booklover? bookworm? There are many tutorials on-line on how to make it safer, but a good rule of thumb that DH taught me is to include lower caps, upper caps, numbers and signs (unless the latter are blocked due to some idiotic page-maintenance scheme). Don't use the same password everywhere and don't keep it in any physical form anywhere (which can be tricky if you're an unclutterer who wants to prepare for a crisis where the family needs your info).
Another tip is to try and log in with https wherever possible. Facebook for instance allows both http and https, and so my bookmark nowadays leads me directly to https ("s for secure"). If you log in with http, anyone who catches your info while it is on its way to the other site can read it and obviously use it.
Edit: apparently KeePass is fairly popular: http://keepass.info/download.html
