I'm a fan of Free. In this world of 'puting I often run into fully functional free things more often than in our tangible world. Where a thrift store will make me pay a nominal fee for those vintage pogs, the tubes throw gifts at you constantly or at least close to the price of those pogs. Obviously the intrinsic nature of of making things with 0's and 1's makes this possible, however in a crowded interweb, many of these great programs get lost in the digital dust.
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1) SIR1 Impulse Response Reverb
Although it's big brother SIR2 has many fancy features and isn't too pricey at about $150, the SIR1 reverb holds most of the same features and a majority of the most important functions.
Like other Convolution reverbs, SIR takes 'impulses', which are recordings that capture the sound of a space, and lets you recreate that atmosphere in your own studio. I bet you never thought your mom's basement could sound like the Vatican, did you?
While SIR1 is a great reverb all around, where I personally think SIR1 shines is in its use as a compositional tool to create brilliant ambient textures when fed longer arbitrary audio recordings rather than the typical brief impulse recordings. Rather than imitate existing spaces, you can create flowing sonic wonderlands from the sound of a rain or a cat fight. You can do this in many Impulse Reverbs, many are not free or as straight forward as this. The file browser in the GUI basically guides you to make strange impulse decisions.
SIR1: Rain & Helicopter Impulse
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2) PSP PianoVerb 
Another very unique reverb. This one "reproduces special kind of reverberation originally provided by piano strings" and seems to pianoize anything it touches. If used with something without pitch like a drum loop, it seems to just spread a wash of random piano harmonics all over it. However when used with something very rich in harmonics, like say a Balanese Gamelan, it injects a stunning world of reverberant harmonics into the sound. This is a really special reverb that has no reason to not be in your VST folder.
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3) KResearch KR -Reverb FS 
This is just a plain jane reverb effect. It features a kind of tacky, synthmaker looking GUI, and controls that are a little awkward. Now the website doesn't provide if they were trying to emulate a certain type of reverb (digital, plate, spring etc) as I would like to know, because all I have to say about this unit is that it is DAMN smooth. It's like my synth is swimming in a creamy pool of vanilla mouse when audio is streaming through this plugin. It provides a deep lasting reverb without getting in the way of your source material. It never gets too soupy or muddy, it just floats along it's merry way in its velvet coated output. Subtle or not here it comes.
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As for a few honorable mentions:
Rhythm Labs Mo' Verb - Solid and very useful reverberator, simulating both spring and digital reverbs.
Magnus' Ambience - Really solid and versatile reverb. Smooth and subtle, or not! 64-bit however a little heavy on the CPU usage.
