One cool feature I've noticed in modern indie games is that in outdoor scenes they like to have a sun that follows the player around, but when the sun approaches the edge you can see the beams from the sun over the top of everything...but once the centre of the sun has gone behind a solid, the sun becomes less bright and stops 'blooming'.
The way to do this (i think) is to have two objects, one which is behind everything (so the sun), and one that is in front of everything (the bloom). You then make them have the same position, and when the centre of the bloom is overlapping a solid you make it fade out, and when its not, fade back in.
I used additive so no pixel shaders are required
Heres a screenshot: