The "Super Quartet" is a rare gem. It is a rack module unit which contains the design/circuitry of a Roland Juno 106, TB-303, and TR-707. It is often overlooked because it has no internal memory, so you can only save patches using patch management software on your DAW. The "Juno 106" portion is carved into two parts: two voices for "melody" and four voices for "chords." You can use a different patch on each. The "bass" section, although modeled after the famous TB-303, uses a Juno 106 voice chip and signal path, so it doesn't sound quite like the TB-303 it is based off of. The "drum" section is basically just a TR-707 thrown in for good measure. The TR-707 is not analog, unlike the famous TR-808, but it does contain some very classic 80s percussion sounds. The MKS-7 makes many appearances in songs I write.
This is an old electric piano module from Roland. It doesn't have a large selection of sounds, but I think all of the sounds it does have are very usable. It has 16 voice polyphony, back when that was considered a HUGE number. It gets 16 voices through brute force. Upon disassembly of the unit, you find the same circuit pattern repeated 16 times over a series of stacked circuit boards. It's big, heavy, and hot. The best part, though, is when its warmed up and you get jamming on it and start hitting keys hard and maxing its polyphony, it has this gorgeous natural analog fuzz! That was a big surprise. I bought it because it was cheap and I figured "why not?" I'm glad I did. I don't use it often, but when I use it, it performs very well.
E-Mu Proteus FX
This is the first rack module I ever bought. It is a good all-around synth with selections of sounds from all different categories. I don't use it often anymore, but occassionally it has just the right sound I'm looking for! The "FX" models have a built-in effects processor. The reverb sounds very tinny, but the rest of the effects can be useful here and there.
The D-4 is an 90's-era drum machine from Alesis (the original Alesis). The best part of the D-4 is its large library of diverse drum sounds. The worst part is that it only has two sets of stereo outputs! They decided to waste the rest of the back panel with trigger inputs. All in all, however, it's a good little drum machine and I use it very frequently.
FM synths are well-known for their unique sounds (especially their deep bell tones) and the FB-01 does not disappoint in that respect. It is very similar to its big brother the DX-7, but can only be programmed using a computer that can send the appropriate SysEx messages and only has 4-operators. Despite this, you can find these units super cheap and they still have a wonderful retro FM sound. The built-in presets remind me of the instruments in the old Sound Blaster / Adlib sound cards (which also used FM synthesis). Although it's not really a "rack mount" but more of a table top synth, I figured it was close enough to stick in this section.