Kingston's first-generation MobileLite Flash card reader was probably small enough, but the successor - aptly titled the MobileLiteG2 - is even smaller.
The multi-card reader, pictured below, is designed to slip into your pocket and measures just 62.15mm x 29.40mm x 16.40mm, whilst weighing 18.9g.
There's support for a number of card formats - including SD, SDHC, microSD, microSDHC, Memory Stick PRO Duo, Memory Stick PRO-HG Duo and Memory Stick Micro - and the unit features retractable covers on each side to protect both the USB connector and Flash memory cards.
Kingston ships the MobileLiteG2 with a lanyard and a short USB extension cable, ensuring you needn't worry about any tricky-to-reach USB ports. It's backed by a two-year warranty, and available now at an MSRP of £4.54.