Um, you most definitely don't want to delete the system frameworks (AppKit is a major one).
Once you delete everything under /Developer, the most space still taken up is probably by the documentation, most of which is stored inside the framework bundle. So it would be safe to delete everything under /System/Library/Frameworks/AppKit.framework/Resources/English.lproj/Documentation (plus similar Documentation directories in other frameworks, Foundation particularly). It's also probably safe to delete the Headers directory as well, but those take up relatively little space. Don't delete anything else unless you're sure you know what you're doing -- it's not worth the risk.