The way I started out was contributing small patches to the Reddit (social news website) code base. The source code can be pretty daunting at first, but once you spend some time with it then it's pretty easy to learn.

Reddit has several of your interests in common. Algorithms regarding voting, built on the the Pylons framework, monitoring systems to make sure the services are up and running. For me, as someone who is still relatively new to Python, Reddit has a good combination of technology I know about and technology I want to learn about. I'd really recommend checking it out.

There are also subreddits such as http://www.reddit.com/r/bugs and http://www.reddit.com/r/ideasfortheadmins that can give you an idea for features, suggest bugs that may need fixing or even just enhancement requests.

Provided that you're nice, the community is very welcoming and open. The admins are all fantastic guys, as well, which is a real bonus.

http://code.reddit.com/ - take a look.