Screen Space Ambient Occlusion, again
Its a lovely effect, but my research has lead me to two possible versions of this algorithm, based on this thread over at gamedev. I must say, there is quite a lot of good stuff over there and I've had fun reading up on it all. The thing is, its a tricky effect to get right and needs a lot of tweaking. Also, the FPS can take quite a big hit because you need at least 2 geometry passes and 1 SSAO pass to get it to work. With that running, you'll loose a few frames. In addition, you need an accurate depth buffer. I'd forgotten this and it took me ages to track it down. GL_RGBA32F_ARB is your friend.
Watch this space, I shall be improving it! :D