Trac is the place to follow along with the development of WordPress. You can track changes in the Timeline section of this site. There is also an RSS feed and a mailing list for those interested.
Bugs reports are also kept on Trac. If you are looking to submit a bug report, please head on over.
You may login to this site using your WordPress.org username and password. If you don't already have one, sign up at the forums.
There are many ways to contribute to WordPress. If you want to get started quickly, grab a bug and see if you can reproduce the problem it describes?
The Life of a Ticket
Reporting Bugs in the WordPress Codex tells how to report a bug obviously, what all the keywords we use in Trac mean, and more.
How to Submit Patches
Check out this great article by Owen on the subject of creating patches and using trac.
Mark Jaquith's Unix WordPress Toolbox
Peter Westwood's Windows WordPress Toolbox