Magnetic Loop Antennas

These look pretty interesting. I have heard of them beofore - you can make a physically small antenna for the low bands, but they also work well for all HF bands. Different designs yield different levels of efficiency and frequency coverage.

Check out these sites for more:
http://www.alexloop.com/
http://www.standpipe.com/w2bri/

AA5TB has a nice list of a bunch of other sites at the bottom of his:
http://www.aa5tb.com/loop.html