You're right Slashhax0r, the standard seems to be vertical for the APs, and horizontal for the cards. I don't recall if it's a 20dB difference, but you can see it on a client's signal meter just by rotating a laptop 90º on the long axis. Of course, getting that great signal has it's price, as typing becomes a real bitch.
A couple of people have discussed helicals, although I don't recall anyone actually using one.
And yes, a fair amount of people have built their own. Take a look here:
http://forums.netstumbler.com/showth...&threadid=2034 Other threads about this abound. Search on "homebrew" and you'll get quite a few.
Of course, the knowledge of the forum members varies from "why won't tv coax work" to people using sophisticated antenna modeling programs, so you'll see a wide variety in skills and understanding.