I just say the Phidgets "Voltage Divider" board and thought that this would make a very handy shield for Arduino. Basically, have 6 (or 8?) variable resistors wired to the analog pins, with easy to use inputs where you can stick sensors that have a variable resistance. It would make hooking up a wide range of sensors as easy as plugging it into the board and adjusting the knob until you get a decent voltage swing.
Here's the phidgets board in question: Phidgets Voltage Divider