I've always liked the idea of the Art NightBassssss, though I've never had the chance to play around with one.
If all you're after is distortion, though, there are other options...
Wounded Paw
WP used to offer the Black Sheep as a rack pre-amp,
but I'm sure if you wanted a WP Attack Goat or WP Battering Ram in a rack format you could get it.
Viva Analog 's 360+ has an awesome distortion to it, IMO...
PAiA Quadrafuzz
"This sophisticated fuzz designed by Craig Anderton splits an instrument signal into four specially equalized channels and fuzzes each channel separately for the smoothest, cleanest sustaining sound with minimum harshness. Fuzz outputs are normalized to a single output but are also available at individual output jacks. Panel controls include separate level and resonance controls for each channel, effects loop send and receive jacks and foot switch input. If you're tired of the same old fuzz and want something that will let you realize an entire palette of distinctive sounds ... here it is."
PAiA Stack In A Box
"This mono tube pre-amp is optimized for use with guitar. Its vacuum tube stages add fullness and warmth while two switchable filters simulate the timbre of speaker cabinet resonances. You sound like you're playing through a stage stack even if you're using a practice amp or Direct Injecting.
And despite its low cost, this double high rack unit also features high and low impedance unbalanced inputs, "bright" and "fat" switches, an effects loop, a balanced XLR output for interfacing with pro studio gear and both normal and inverted unbalanced outputs. Best of all, this Craig Anderton and John Simonton collaboration just sounds WAY COOL. Wall mount power supply included."
All the aforementioned is available as new, I think.
Another option for new: Broughton Audio might do up a rack unit for you if you send TB member Azure Skies a meme.
Hunt the classifieds for USED GEAR...
RAT R2DU
MXR Omni
You could also search for an older Line6 rack Pod,
or the Digidesign rack interfaces,
Fractal's fricken Axepensive modelling,
the Chandler Tube Driver dealio
Thermionic Culture Vulture is quite popular in studios:
Then there's the ubiquitous Sans-Amp stuff.
Try searching other forums,
too.
Also look at modular synth rack gear, there are some hidden gems to be found out there in synth land. The 500 series stuff is gaining popularity with ODs and even Muffs available ...
Lastly, why limit yourself to rack gear when you can take any stompbox and stick it in a rack drawer?
TBer Bassdokes setup a few years back: