The Tuff-N-Tiny drives would seem to be about as small as such drives can physically get, being no more thicker than a penny, with thickness of the USB tongue itself.
The Tuff-Clip is a bit meatier and has a standard enclosed USB tongue. The key features are an integrated carabiner-style clip and a retractable slide mechanism for the connector. An ABS plastic casing adds additional protection.
Both drives are compatible with all major operating systems, including Linux. Both are USB 2.0-compatible and available in 4 GB and 8 GB sizes.