One good piece of donationware that I don't see mentioned often is
Rhythm Lab's Mo'Verb: http://www.rhythm-lab.com/plugins/mo_verb
the stereo image controls are a nice feature.
Surprised to see no mention of TAL's reverbs; I seem to use TAL-Reverb-II the
most out of the three.
I loved Gregjazz's Cinematic Reverb (especially for unsubtle ambient bigness), but ended up having too many stability issues- since OP is on XP, this may not be a problem.
For other "more ambient-dedicated and less general-use" reverbs there's
xoxos' Waterverb and HomeGrownSound's SoundScaper II.
Also agree heartily with previous replies- SIR, Ambience, GlaceVerb, Reflex Free, Abstract Chamber, Freeverb(s), LiscVerb, etc. are all quite good with their own strengths.
Swiss-army-reverb Space 360 (when behaving) is amazing (especially automated).
If more links are needed for what I've mentioned other than Mo'Verb, I'll edit the post to include them, but they're not hard to find.Can't help but mention a little trick: if looking to add some retro flavor, try adding
a little spring reverb before your main plate/hall/etc. reverb; just don't forget
all the freeware spring reverbs are mono, so some routing twidgetry might be required if preserving the stereo image prior to the reverb(s) is wanted..
Shutting up now.