It's not well-known, and we haven't hacked on it since 2001, but it works pretty well as is. And it's GPL. It requires no installation, and the zip file is only 117kB.
The sourceforge page has since lasped, but here's the last version we released (with source): compass.zip. Try it out; if someone wants to do more with the code, by all means.
"You stay here, Audrey -- this is between me and the vegetable!" -- Seymour, from _Little Shop Of Horrors_