Welcome to the elgg.org developer center. If you are interested in developing on the Elgg engine, then this page will point you to all the resources you might need.
As always, if you like what you see with Elgg then please consider getting involved in its development.
Here you will find information about Elgg's architecture, approach and features.
This is Elgg's bug tracker, if you find bugs or have patches for existing bugs then head on over and get involved.
Developer mailing list
This mailing list is aimed at experienced developers, not end users.
This is a handy resource to search and find out what functions exist within Elgg.