I did a post on how you could expand the storage on your Nexus 7 through a neat little wire and an app. Well that little wire is a lot more useful than most know.
USB OTG cable is a neat little MicroUSB to USB conversion cable. This cable will allow you to connect most USB accessories such as SD card readers, keyboards, controllers and portable HDD.
The only problem with it is that it's hard to work out what devices are compatible. It looks like most of the Samsung family of Android devices work. Others include the Nexus family, Droid RAZR and some other major Android tablets.
With the cable only being around a quid or two it's worth trying out anyway.