One that I've been playing with lately is the KR Reverb that comes on the Computer Music cover DVDs. Pretty cool and tidy reverb, but no presets so you have to learn on your own.

My favorite from rharv's list is Ariesverb. There's a pay-for version, and free 'alpha' versions. The alpha works fine. http://www.ariescode.com/index.php

Keep in mind that this reverb has controls unlike most. I would start out on the Kjaerhus classic reverb first. Download the whole classic series from the acustica site here (all of these effects are worthwhile to mess around with and use in your projects) http://www.acoustica.com/plugins/vst-directx.htm

-Scott