I've had great luck with convolution type reverbs, Altiverb is one that comes to mind.
The best "ambience however, comes from well placed instruments and well placed room mics. I'm a fan of compressing the crap out of selected room mics, while I know that others may shun this technique. Put up a few room mics, be creative. I usually walk around the room and listen for different sounds and then put up a few mics.
Never discout well timed delays, I find that a well timed and carefully mixed slapback on vocals, guitar and even snares can really add a touch of ambience and space to a record.