Recently I've noticed a couple of projects on github that extend the functionality of Xcode 4 via plugins.
Two projects as examples by @olemoritz:
Both projects are installed into ~/Library/Application Support/Developer/Shared/Xcode/Plug-ins and Xcode just picks them up.
Are there any sources of documentation (officlal or user generated) on extending Xcode?
Edit: ping @olemortiz ;)
As I wrote those plugins you mentioned, here are some pointers:
There is no official documentation from Apple, so while Xcode does have a plugin infrastructure, it is entirely private API. (but hey, no one wants to submit Xcode plugins to the App Store, right? ;)) – The usual warnings apply: You should code very defensively, and it's possible that Xcode updates break things. Any plugin can bring Xcode down entirely, so be careful.
There is a seemingly abandoned effort to document the plugin interface here.
There are some open source projects that allow you to see what's needed to get a plugin loaded at all, e.g. mine and there's CLITool-Infoplist (I think that's where I got the basic structure from, but I can't really remember, because I've been doing this without publishing anything for quite a while).
You can use class-dump to generate headers from Xcode's private frameworks, e.g. IDEKit and IDEFoundation (in Xcode.app/Contents/Frameworks). Reading those gives you quite a bit of information on how Xcode is structured internally. DVTKit and DVTFoundation (in Xcode.app/Contents/SharedFrameworks) can also be useful to class-dump.
You can observe all notifications that are sent in Xcode by registering an observer for nil. I initially just logged all those notifications to get an idea of where I might be able to hook into.
Good luck!