Discussion on the Muff Wiggler modular synth nerd forum (multi-page / single page) of the SID GUTS module for Eurorack, which incorporates a vintage SID (sound interface) chip from an old Commodore 64 computer. The module is designed to port that characteristic lo-fi SID sound into the analog hardware environment and sounds great, but cuts off much of the functionality of the chip if you have the Commodore (and a tracker program, as Nullsleep notes on the forum) or an Elektron Sidstation synth (an amazing device, although Not Truly Hardcore in that a savant in Sweden did much of the programming for you).
The SID chip has 3 oscillators, each of which generates a variety of waveforms. In the Sidstation, and presumably trackers, these can be programmed as wavetable synths, giving rise to the familiar, still very desirable, arpeggiated arcade sound.* SID GUTS uses one oscillator as its main voice, a second oscillator to modulate (only) the triangle wave of the first voice, and ditches the third oscillator. You could buy 3 SID GUTSes to get a chord (as one Wiggler pointed out), but you still wouldn't have access to the wavetables. You can do a few interesting things with control voltage inputs to tweak the SID GUTS' sound, such as using an LFO to toggle back and forth between ring-modulation and "sync" mode, or to switch between waveforms in real time. And of course you're getting the SID chip's analog filter, which some Wigglers think is divine and others a throwaway.
The biggest drawback to the SID GUTS (which possibly had a limited run and may not even be available at this point) is that, unlike the Sidstation, the module doesn't come with a SID chip! The guts, if you will. You have to fish one out of an old Commodore or shop on eBay. There were a few different versions of SID chips made, and again, the Wigglers are divided on which is best. So you could spend $400 for the SID GUTS and still find a bum chip and not get the best use of the thing.
*A "long-time SID programmer" on the forum asserts that the SID chip "these days... can produce extremely sophisticated sounds, you would not believe (way beyond than these blip-blops). Even, there are more ways to trigger a voice and all sounds different (hard restart alone is an important sid-sensation factor). Wave-scanning-tables are also super important."