The Betanews website lists loads of beta test programmes to get involved in.
Another site listing new programs ready for beta testing.
This page, on the excellent shareware marketing resources site has a list of beta testing sites
If you want to test games then this is the place to visit
This company offers to manage beta testing for software developers and will put you in touch with companies looking for beta testers. They have a hosted beta community programme.
BetaSphere will put you in touch with companies looking for beta testers.
Check websites of companies you respect
download the software try it report defects professionally mention that you would be happy to help test future releases
an article about how to become a microsoft beta tester
This always pops up interesting stuff
The analysis by Forrest Cavalier is silent on how open source testing is better than closed source testing. He mentions a number of "reduction factors" which apply to bazaar (open source) testing, but these same factors (and more) also apply to cathedral (closed source) testing.
essay on beta testing might be useful
A book on beta testing to help companies conduct their beta testing programmes better.
What one company wants from their beta testers.
Loads of new software listed every day for you to help test.
Open source software ready to beta test.
GNU software homepage