Beavis' simple Flooper is the classic, I like his one with a little more control though, such as the Beavis-one in the article at the bottom of this post.
There's the Ohnoho's "Utter Stutter" which IIRC combines a feedback looper with a tremolo.
As for bells and whistles on a flooper...
Here's a few ideas I've messed around with for the FFFX Super-Flooper, use what you like but maybe you'll be inspired and come up with some of your own twists.
stompers:
ON/OFF (bypass)
FLOOP
100%WET ie DRY KILL
+ MOMENTARY for DRY KILL or FX so dry continues while fx cuts in/out vice versa
STUTTER
ON-OFF ANTI STUTTER — have this patch-bay wired so it can do the whole signal or just the effect
ANTI-STUTTER [ this is greatly simplified to just two stomps now]
FX LOOP SIDE INSERT FLIP-FLOPPER
WARP!
KNOBS:
- Wet
- Dry
- DRY Gain
LEFT SIDE FeedBack
RIGHT SIDE Feedback Vol
AND MAYBE ...
BuffNBlend stomp & knob
DON'T forget HPF & LPF ! These don't have to be toe-controlled, could be small 9mm switches.
LPF on CLEAN
HPF on FX
Other considerations:
A limiter/compressor for after the feedback section of the Flooper, to help tame volume jumps and squeals
A GGG Translocator to swap in/out additional effects into the flooper such as a Hard-Chop tremolo (Vox Repeat Percussion), a Bitcrusher or similar or Escopedo's LoFoMoFo, a Parasit sonic reducer something or other
Buffered Bypass option for tails in case you put a runaway delay in the flooper...
Devi's LDR concept has already been mentioned by Coltonius, above, but you could also consider adding a proximity sensor ala ZachVeXx's Probe Wah ...
or via an optical sensor like the robot-builders use:
That way you could control the amount of feedback going through the flooper via a foot, and infinitely more cool than a simple BIG KNOB, or S
LIDER that breaks the first time your clumsy-oaf size 13 foot hits it...