One of the Apple's more polarising design choices with its mobile devices is the lack of a slot for an microSD card or similar avenue for expanded storage. Yes, you could just buy a non-Apple tablet or phone, but if you're just too attached to the hardware from the big white citrus, you should check out the iFlash Drive.
Simply put, it's a flash drive that provides a USB and 30-pin / Lightning connector so you can plug it into your iPod touch, iPhone or iPad as well as a conventional PC.
There are, of course, a few conditions. To move data on or off your device, you'll need to use an app provided by the manufacturer. As for reading the drive with your PC or Mac, it operates just like a regular thumb stick.
If you want to pick one up, you can head over to Photojojo, but the price might put you off. The 8GB, 30-pin version is $US99, while the 16GB with the Lightning connector is $US149.
The difference, other than size, appears to be a removable Lightning adaptor on the 16GB, so if you already have one, or are happy to pick one up, you can just go with the 8GB device if you don't need the extra capacity.