Malekko Voltage Block: 8 offset generators. You can sequence these generators, each with separate lengths, clock divisions, quantization, etc. You can also jump to any stage, hold a stage, or arpeggiate a subset of stages by using the stage buttons. Even if you never use the sequencing capabilities ( Shame!), the 8 fader offset workflow can feel somewhat similar to a controller like the Faderfox UC4. In my opinion, this is the single best 301 partner.
Monome Walk: This is key for the Looper unit. This uses two footpedals to generate six gate outputs: two momentary outs, two toggled outs, and two logical outs based on both pedal states. I love that the momentary and toggle outs are available simultaneously instead of existing as separate modes. Bonus: It's cheap and 4HP.
Intellijel Planar: There's the obvious X/Y joystick out, and the joystick is large and awesome if you haven't used one. Less obvious is the fact that this works really well as a 4-channel vector mixer, making this a perfect way to scan the four outputs of the 301. (EDIT: Alternatively, you can use the 1-to-4 capability to take one input and smoothly distribute it to the 301's four inputs for variable processing)