Alesis Nanocomp
I haven't used one but from what I've read I wouldn't pay more than $50 for one. It could work just fine depending on your needs/expectations.
Those are cool little cheap compressors with the proper controls a compressor should have. Great for learning how to use compression. A little awkward form factor to use as a pedal, best kept racked.
My church had one in our PA rack for the pastor's insert channel. It did the job, but was noisy when the compression kicked in, and it just recently took a dive. There are better cheap compressors to get.
+1 to the above
Sounds like you are not looking for a pedal unit
I hate to say it, but a used Behringer rack mount is a great learning tool and works well
They are around $100 new and 1/2 that on the used market