At the same time I ordered the Behringer ARP 2600, I also ordered a Behringer Neutron. The Neutron (like the ARP) is a semi-modular synthesizer with a large patch-section in the right side. It does however "only" feature two oscillators, and it have 2 envelopes and a single LFO (well the 2 oscillators goes full range, so these can double as LFO's also). The patch-section is very feature rich both with summing and attenuation and many other features.
While the Neutron have a very "fat" analog sound on its own, my primary reason to get it, was to use it to further modulate the ARP, by patching between the Neutron and the ARP (I dont have any Euro-rack equipment, so this "what I got in stead"). But in some cases the Neutron will be used by itself. I also bought the rack-ears for the neutron so both the Neutron and the ARP will be mounted in a rack with the Neutron sitting just below the ARP for easy patching between them.
The default red-overlay on the Neutron is missing some descriptions for the new features added in firmware 2.0, and I am not a fan of the "background" they have used in the patch-section (as I think it makes the labels less readable). So I ordered a new/3rd party overlay (from Heina Kroon). This (very colorful) overlay contains description (labels) for some of the features added in firmware 2.0, and the corresponding input/output patch-points in the patch section are colored in the same way, so simply looking at the colors (without reading the labels) you can quickly identify the patch-points to use. A lot of different overlays are available from Heina Kroon, but as I said I chose this one, as it allows me to quickly identify the various "modules" of the synth and the patch-points to use (I prefer practical over pretty).