Proto Screw Shield kit. This shield makes it very easy to connect sensors to the Arduino, and still be able to stack additional shields on top of this one. There is quite a bit of room to put additional components on this board as well. The kit has to be assembled, which takes about 15 minutes, and is easy to do. There is a good tutorial for this at https://www.sparkfun.com/tutorials/266
The above shows a SainSmart LCD Keypad shield stacked on the Sparkfun Proto Screw Shield, stacked on a SainSmart Uno. All the UNO pins (rev 2 and earlier) are exposed.