Screen Space Ambient Occlusion

Real ambient occlusion is too heavy for even modern computers to perform in real-time despite their vast processing power, so techniques that produce something in the right direction without bothering with most of the real calculations pop up (like SSAO).

A noticeable problem with SSAO is that it produces a halo (usually black but can be anything really, though usually some color used by local or ambient lighting) for objects in certain lighting conditions.