The ExpressCard/34 port is an expansion slot that can be used for many purposes. Media card readers (I have a SIIG 11-in-1 that works great), broadband modems (cellular internet data connections), eSATA adapters for faster connections to external hard disks, solid-state drives, additional USB/FireWire ports, etc.
See this page for a listing of ExpressCard modules (but note that not all will work on Macs, and also that there are two sizes - 34 mm and 54 mm, and only the 34 mm cards fit in the MPB's slot.
http://www.expresscard.org/web/do/pub/resourcedirectory
Or, if you don't need it for any of those but you do use the IR remote for Front Row, there's this IR remote which fits into the ExpressCard slot for storage:
http://www.sik.com/rex.php