Free reverb? Ho-hum.

Free reverb that can simulate bizarre acoustic spaces and even make your mix sound like it's underwater? Now you're talking.

Dasample Glaceverb
(VST Windows) is a Residual Vector Modulation reverb plugin — think
physical modeling applied to reverb. With control over every parameter
of the space and its materials and 64 presets, it's capable of
everything from traditional stage reverbs to bizarre boomy metal boxes
and underwater effects. And it's completely free, so if you're on
Windows there's really no reason not to be applying odd, boomy 20s
reverb tails to your stuff within moments.

It sounds a bit like a spring reverb from an alien culture. Have fun!