There are also a few techniques that produce approximate results - things like rendering with particular blend modes to accumulate total coverage at each pixel, then using a heuristic to approximate the actual alpha of each primitive according to it's location along the min/max depth ramp, or some other simple-to-compute value. I'm not sure any of these have been published, but I've worked through one myself, and I've seen other folks discuss them. These tend to require a second pass, though...

Edit: Just to provide some examples... Check out http://www.houmany.com/htmls/grafx.html (Sort Independent Alpha Blending), and http://developer.download.nvidia.com/SDK/10/opengl/src/dual_depth_peeling/doc/DualDepthPeeling.pdf (Ignore the Dual Depth Peeling stuff, and check out the 'Single-Pass Approximations' section at the end.