There were a lot of articles and forum posts about this technique. It was very popular and eventually received variant modifications. The main idea of Screen Space Ambient Occlusion is to calculate local ambient occlusion as a post-effect using a scene depth buffer and a normal buffer. You can add this technique to your engine very easily. It doesn't require anything, except two buffers.
I compared 2 implementations in a Demo. The first is SSAO from Crysis with some modifications, and the second one is a ported version HDAO for DX9 from DX10 SDK. The second looks much better, as you can see in the demo.
