PSP PianoVerb is a slightly quirky, but very nice sounding reverb. It is available as VST, RTAS and AudioUnit for PC and Mac. It is freeware, although it does require you to authorise it. Product page and download here.
PSP PianoVerb reproduces special kind of reverberation originally provided by piano strings. It generates reverberation using twelve string operators with adjustable damping and decay time. Each of them is tuned to a particular note starting from A(55Hz) to a G#(103.8Hz). The PSP PianoVerb string system can be transposed in the range of +-24 semitones, tuned within a range of +-100 cents and be detuned as well. Transposing to lower octaves produces a bigger and less resonant sound similar to traditional reverbs, while in turn transposing to higher octaves produces a very bright harp-like effect.