For people doing native development, RenderMonkey is an invaluable tool:

http://developer.amd.com/resources/archive/archived-tools/gp...

Really great way of prototyping shaders, though I don't think it supports geometry shaders right now, and it seems to be only Windows.

For debugging, gDebugger is amaaaazing (and has Windows and Linux support):

http://www.gremedy.com/

It lets you inspect textures, buffer objects, and suchlike, and can help you profile graphics calls.