Wizooverb, if you can find an old copy. One of my go-to reverbs along with Ariesverb. Great sound, great GUI. Wizoo is gone, and no one took it forward. Algorithmic and "High Def" Convolution. Cute trick: Convolution early refections fading into an algorithmic modulated tail, but you can get one or the other purely if you want. Sounds terrific, but more clinical than Ariesverb. I probably haven't played with it enough. I think it has modulation somewhere. Much, much more approachable than the latest Ariesverb which lets you play with the algorithms. Won't ever go 64bit, obviously. Where the heck is the code? It should be released, not chained. I'm an Open Source hacker.
Ambience. The go-to reverb for many people, but for some reason not me. Lots of people agree with the tag line "Rivals the quality of many commercial reverbs." Lots and lots and lots of great presets. Get it for this alone. Among the preset developers you'll see Rob Papen who is rather famous in the plugin world. Hey, I'm liking the presets right now as I write this.
MuVerb. Algo. From MuTools, makers of the MuLab audio production workstation. Original algos, not Synthedit. Really nice tails! Odd, but nice purple GUI. Seriously, this is a nice reverb and I need to start using it! Watch me put this up near the top of this list.
Reverberate LE. Convolution with many controls. Commercial promo-ware which probably gets updated.
Old freebies that are still useful, but will probably stay 32bit:
Epicverb from Variety of Sound. He's still in business and has other great stuff. This is the go-to freeware reverb for many people. Algorithmic. Get the presets that are in another download. Nice GUI for a freebie! Not too many controls, but I have trouble controlling it.
Revolverb Lite. Convolution reverb for people who are non-technical. Three total controls, plus loading your convolution file. Sounds great, just like you'd expect a good 16bit convolution verb to sound like--despite the dated GUI. Was this the first freeware convolver? There never was a non-Lite version. Just get this. You'll like it. Download a few impulse response files and go to it. Search the forum (not the whole site) at kvraudio.com. This verb does not get enough attention.
SIR is the commonly-mentioned freeware convolver, but since it went commercial for V2.0, it's really in the same boat as Revolverb, but it works and offers more controls than revolverb.
EVM V90. I don't like all his plugs, but I have some affection for this one. Maybe it has some fixed modulation in the tails. Can go to huge hall/outer space sizes. Simple, cute, algo-only.
Gigaverb. Algo. Nobody but me seems to use it. I mentioned huge halls in EVM 90. Gigaverb is for when you really need spaces up to 300 meters in size and decays of up to a minute. Smooth! Good to have in your collection. Hard to find, get it here: http://www.mathieubosi.com/zikprojects/GigaverbVST.zip
Antress ModernSpacer (black or silver) The Modern-Foo plugins, and there are dozens, are a mixed bag, but this is a nice little algo reverb. Algo. Sounds pretty good to me. Hmmm, a quick check says it doesn't appear to be a Synthedit-made plug. I don't like his other verb. Remember that (pace Waldorf, Inc.) black sounds better.
Cinematic Reverb. Algo, ancient GUI. Good sound, and can go into nearly infinite decay. I love infinite decay.
Freezechamber. Algo. Speaking of infinite decays... This thing has a freeze button which keeps the reverb sound from decaying! I think it's just looping over the buffer, but with some smoothing so there's a clean loop. Doesn't sound like a loop, though. This is a serious product. Has manual, has MIDI controls. Just get it.
Empire 2. An oldie from the well regarded spiritcanyonaudio.com. Nice sound, you get to select from the algorithms, which have helpful names like "Catacombs" and "Ballroom." Ugly but usable GUI. A professional product, really.
Hispasonic reverb, hispverb, Hispasonic. I don't know why this algo seems to sound better than the Freeverb it's based on. I think this is just a skin, really. Maybe it's the latin flavor. It has the "Olga synth" factor for me. I can't read the controls and have to experiment. Algo, the Freeverb algo.
Freeverb. Algo. Many people like it. I don't use it or the Hispasonic version. Ugly but usable GUI. Has a convo version that I have not tried. Now has 64bit. In active development.
LIŚĆ VERB (hey,with a name like mine, I can type this name correctly) Brand new. Synthedit. Is this the standard Synthedit reverb? Doesn't sound bad, and the GUI is rather nice. I want to like the guy whi makes these "Saltline" plugs. Has a weird freeze implementation that is hard to use, you have to turn a knob. I don't love the sound. Maybe that's just me? I haven't seen any reviews.
Mverb. Wusik did a new GUI for this classic algo. Get it from their site. I like the "quad" version which gives you two reverbs in parallel in one plugin. I used that for flexible early reflections using one, and tail from the other. See the original Novaflash-style GUI on the single version and recoil in horror. No offense to that artist, I just don't get his GUI designs. I just picked up the 64bit versions since I was writing this.
Reverbering. Rather old, but I like what I heard just now. Bizarre GUI, good sound, very few presets--but nice ones.
GlaceVerb. A current favorite of many people. Nice sound, seriously great GUI, with a checkerboard display of the room you've created. A professional product all around. Can't go wrong with this one. See their new synth plugs, too, but I have not tried them.
Reverb-O-Matic. Fun! Moorer's six comb filter algo. File name moorereverb differs from the plugin name I seen in Reaper (Reverb-O-Matic). You can get wild interacting chaotic echos with the six LP coefficient sliders. Eccentric GUI. Wear sunglasses or ingest substances before opening. Wild fun!
RoomMachine844. Algo. Old GUI looks like a drawing of a rackmount. Decay control seems to operate backwards. Some people love this thing and it doesn't sound bad. It was a go-to for a while in the freeware scene. Original algos.
Space360. One of those interesting reverb plugs that show you the parameters as speakers and a human head in a room, but, no, you can't drag the icons. I can't get good results right away with this and I dropped it. You might do well with it.
Last but not least: Algorverb. Simple, clean GUI. Really nice sound. Original algos. A forgotten gem. I love this one. You can push it into oscillation, and infinite decay. A grand total of four presets, but it's so simple that it's ultra easy to make your own sounds. Six total sliders. No other controls. Mayhem if you want it.
That's all folks. That's the contents of my Reaper reverbs folder. Try not to ask me about my old EnergyXT reverb folder. It's scary in there.