I'm rather partial to the
Sony Memory Stick Pro Duo since they are also used in my Sony PSP and Cybershot camera and no adapter is necessary. I've had no problems with the 1GB, 2GB or 8GB sizes in my reader. There are also slots in my PC to accept most types of cards, if not all.
With all these new super duper sizes coming out, they are clearing out the lower capacity sticks at decent prices.
I just bought 2 new 2GB ones at Staples (in-store clearance bin) for 10 bucks each. That's enough to hold all the books *I'll* ever need, plus an extra to back them up onto.
I like to buy my memory sticks from a reputable store, hoping to avoid any counterfeits.
(Anyone is welcome to chime in here with a better price or preference)