In my search for one that was fairly simple and wouldn't take up too much room in a pedal, I came across this article from a few years back by Paul in the Lab. It uses just 3 transistors, a momentary switch, and a DPCO-type relay. I've drawn his circuit up for use with a board mounted SPST momentary off/on footswitch (if you need a perf layout, check out this one over on lvlark's blog). You'll need a 5 volt Omron G6S-2 relay (I got mine here), and a lot of different transistors can be used. BC547 and 557s are called for in Paul's schematic, but 2N3904 and 3906s or similar can be used as well. Most SPST momentary switches I found had the solder lugs on the side, so the board has been laid out to fit vertically in a pedal enclosure. Everything is mounted to the circuit board on the component side and the board is small enough to fit in a 1590B.
This has been updated to reflect the discussion below.