Fill out more stuff here about other aspects of Cydia that may be interesting to developers.
For information about running a repository, see Repository Management and Packaging.
TheiPhoneWiki has a list of Cydia error messages and what they mean.
Contributing to Cydia
If you're interested in contributing improvements to Cydia, the process is similar to contributing to WinterBoard. The first step is to get in contact with saurik and Britta (preferably via IRC in the #cydia channel on irc.saurik.com - see How to use IRC) and discuss the idea you have in mind, or explain your skills and ask for suggestions about what to work on.
(If you'd like to help with design, please don't start by showing Photoshop mockups - they generally can't be integrated in a practical way, since they usually don't address all of the constraints that Cydia is working under. Instead, it's helpful to start by explaining that you'd like to help with design, and show some examples of work you've done and what kind of design you like to do.)
After discussing your idea with him and making sure it makes sense for Cydia, saurik usually doesn't need a full code patch from you; if you can provide a proof of concept with a bit of useful code (or a bit of useful HTML/CSS), he can adapt that into the codebase in a way that makes sense for long-term maintenance.