Here's a fun knot I worked out. It's essentially two slipknots securing each other. It forms a secure, single loop, and can be quickly released by pulling on the working end. I'd be interested to know if anyone has seen it used or know if it has another name. Warning: I'm not endorsing this knot as being useful for any particular purpose, use at your own risk.
Update: Thanks to the friendly folks on the International Guild of Knot Tyers Forum, I've been able to identify this as a variant of the Mooring Hitch. The bight in the standing end is replaced by the loop of a slipknot, which fixes the position of the knot on the standing end, allowing it to function as a loop as well as a hitch.