Potentially the worst bug
in the entire history of the world wide web 
If you want the details:
http://heartbleed.com/
(this web page was made by the guys of Codenomicon)
Btw, it is better to wait for the release of a patched implementation, otherwise changing your password is perfectly useless...
And for password managment I strongly raccomand the use of a dedicated software (a password manager) like KeePass
Besides remembering your password it has a very useful password generator tool so you can generate strong password.