Here's a very interesting free plugin: SIR from Christian Knufinke
Download it at:
http://www.knufinke.de/sir/index_en.html
"SIR" is an Impulse-Response processor - you point it to an impulse file (WAV containing sound sample of a room, cathedral, hall, whatever), and it analyzes the acoustic signature of that room and re-creates its natural reverb in your mix. I tried it with the included "cathedral" sample, many other impulses are downloadable. Easy to use, totally cool!