I was recently supplied with some excellent new flash drives by USB Memory Direct. Many designs are available but I chose a credit card sized flash drive to maximise space for images. I was also able to include all my contact details, so the drives double up very nicely as business cards.
I'm now using these drives during workshops to store clients images on once I've demonstrated techniques in Lightroom and Photoshop, as a reference for when they get home and start processing their own images. With a capacity from 256MB to 64GB the drives can have space for plenty of image files.
This drive design is called "Card Flip" and I think it's the best choice for photographers in particular. The quality of the drive seems to be very good and the print quality is also decent. The drive itself is only very slightly thicker than a credit card, so it is easily stored in a wallet. I have been very impressed with them - far more useful than a standard business card and much less likely to get tossed in the bin!