what is it ?
a korg volca ? a monotribe ?
a sweet analog delay pedal ?
perhaps a cheap '80s midi sequencer…
a cassette tape machine ?
reverb rack ?
what do you love to use with your Elektron device(s) ? what small device would you miss ?
I bought a Korg KO2+Monotron Delay for $140 last year and had a blast using the mono to process the KO, playing the audio out of an old practice bass amp. I recorded jams using an old cassette recorder, which I still use sometimes. Then, an Octatrack happened and all nearby minds were blown. I’d really miss the Monotron Delay; it’s too fun to sample.
Arturia Microbrute - sounds quite good for the money IMHO.
Last week I bought an Akai MPX8. 100 bones, not a whole lot to it. Sample play back functions, but it alleviates some rudimentary duties from the OT.
Also picked up a Mooer Shimverb reverb pedal. Love it. Very unique character to it. 80 bucks
Alpha Juno-1! You get a quality sounding analog synth for basically nothing with that. 
Old Korg guitar FX boards. Dirty dirty dirty.
The MeeBlip original was fun. I miss it.
When it comes to cheap kit I love my Freebass383 303 clone - I got it off Ebay for about £90
Its a 1u rack unit with a single VCO that does a decent job of mimicing a 303. Its great fun whacking it through a distortion box and making an acid racket via the Octatrack and MD
+1 for microbrute. fun lil bugger
roland mc-09 for me, hands down.
fun rhythm sketchbox and 303-emulator monosynth with a 2-bar sequencer, plus four 2-bar length loop samplers you could record to, affect/eq and resample on the fly. a reliable 2-trick pony in a live setting. even wrote a performable solo track on it once.
i’ve gotten more mileage out of other gear, even other inexpensive gear, but for $150 i couldn’t believe how good it was.
I use 2 of these run into my MDUW+. 1 is a 4osc board with a pitch knob per osc and a volume knob. The other is 2 osc with filter and pulse width. All of his boards have light sensors on them that can be toggled on/off and they get really crazy. The light sensors will alter pitch, filter, pulse width, etc depending on how you have the switches set up. They can go from plucky and contoured to straight harsh noise in seconds. The light sensors are quite sensitive and even ambient light from TV or anything in the room will alter them slightly. For live shows I use my phone, other led lights, and LEDs from other gear to modulate them. We’ve even used fire which lends to a very organic modulation unlike anything else. They can not be triggered or synced, but running them into the MDUW means you can use the plocks and other things as gates and further modulation. They are DIY boards that I believe cost 65$ As a kit and something like 85$ assembled. Damon (the owner/maker) is a great guy with a ton of knowledge as well as an avid elektron user. He also has recently made his own modular format that is truly quite impressive.
another shout here for the mighty Microbrute. It’s almost criminally good for the money. I don’t really know how they’ve done it to be fair…
Without chatting, the Monotribe. Easily…
MBase11 
volca beats is a good compliment for my octa
EHX Octave Multiplexer.
very nice analog one octave below, follows the envelope.
economical too, sub-$100 range.
going to place this in the envelope loop chain of the QTron+, to achieve a synthesizer-like tone on an electric bass signal.
FMR Audio RNLA compressor - suckin’ on my MD’s stereo out Yeeeeah!
Roland JX-8P - very underrated synth, sure its envelopes are slow but if you want thick creamy pads and soft buttery sounds its king.
Volcas are unreal bang for the buck. I prefer the Bass over the Keys but I have both.
My only complaint is that it lacks a midi cc for the cutoff so I can’t control that via my OT.
Alesis Quadraverb.