Olimex have brought out a new board which allows the Raspberry Pi to have access to UEXT modules like Relay modules, Thermocouple sensor modules, 3-axis magnetometer and accelerometers, Zigbee, WiFi, GSM, GPS etc.
You can purchase one here: RPi-UEXT.
This is a fantastic little board that works not only to give access to UEXT but also breaks out the GPIO pins very neatly. At only a little under 4 euros, you can’t really go wrong – it’s cheaper than the similar Pi Cobbler boards.
The Olimex blog post announcing it can be found here and there is a post looking at the release announcement here.
A comprehensive look at what you can do with the board is contained in the original pre-release announcement.
Looks interesting this one. It is cheaper than the cobbler, but you need to buy the ribbon cable separately (whereas the cobbler includes one).