Akai AX-60 - (typically around $300)
Akai were such a dominant force in the sampler market, that many people don't know that they made synths before sampling came along. And actually quite good synths, at that. The AX-60 was a six voice analog polysynth with a nice interface that people often compare to the Roland Juno-106. The AX-60 was actually quite a bit more flexible than the 106, though, as it included ring modulation and a number of other features not found on the Roland synth. Really nice resonant sweeps and a unison mode for super thick sounds. Also available in an 8-voice version called the AX-80.
Arp Axxe - (typically around $300)
This single-oscillator monosynth often gets overlooked given its relatively simple architecture, but everyone I know who has owned one absolutely loves it. Sure, it's not going to give you Arp 2600 levels of complexity or variety, but if you crave some of the famous Arp sound, this is probably the easiest entry point. There are two versions of this out there that sound a little different. The black and gold version has a bit more of a 'Moogy' filter, while the black and orange version is more typical of the later Arp sound.
Cheetah MS6 - (typically $100-$150)
Cheetah was a relatively short-lived British company and the MS6 was sort of their version of the Oberheim Matrix-1000. Eerily so, actually. The big advantage the MS6 has is that it is programmable right from the front panel, whereas the Matrix-1000 needs to be programmed from a software editor or a Matrix-6. To my ears, the MS6 actually sounded better than the Matrix. It just seems to have a thicker, occasionally almost "evil" sound. It can do some amazing synth strings and despite the envelopes not being super snappy, it can do some nice, meaty basses too.
Korg Poly 800 - (typically $100-$250)
The first synth I ever owned. The Poly-800 combines digitally controlled oscillators with a single analog filter. The single filter wouldn't be an issue, except that this is a polyphonic synth (4 voices if you use two DCOs, 8 if you only use one), which can cause some odd retriggering on sustained notes. It can be kind of an interesting effect, though. Practically speaking, you really need both DCOs to get the best results, as even with the built-in chorus, a single DCO by itself sounds pretty thin. Not an amazing synth by any means, but it can do very nice synth strings, pads, and simple basses. A Mark II version that included a built-in digital delay effect is also available. A rare reversed keys version was also available for the Japanese market only.
Sequential Circuits Six Trak - (typically $300-$400)
While it pales in comparison to Sequential Circuits more famous Pro One and Prophet 5, the Six Trak is capable of some really nice sounds. Six analog voices that can be stacked in a unison mode for more beastly sounds. It generally needs a bit of processing to get the best out of it in the poly mode, but nothing that a little chorus can't sort out. I've found the build quality of a lot of the old Sequential stuff to not be quite as robust as some other manufacturers, so be sure it's in good condition before you buy.
Yamaha TG-33 - (typically $150 or less)
Basically a slightly more beefed-up version of Yamaha's SY-22 keyboard, the TG-33 is a desktop synth that represented one of the first vector synths on the market. It combines 12-bit sampled sounds and FM and you can crossfade between layers using the joystick or various modulators. 8 part multi-timbral too! The biggest disadvantage it has is that it lacks a resonant filter. But if you can get past that, this can do some absolutely stunning pads and atmospheric sounds as well as giving you access to vector type sounds without having to spring for a Prophet VS or Wavestation.