By now, you're probably familiar with some camera sensor sizes. APS-C and full frame are probably old hat by now, but venture into the world of compact cameras and things get pretty complicated. That's where Sensor-size.com comes in.
The interface is as simple as the concept: You type in the sensor ratio or mm measurements and it spits out the diagonal measurement of your sensor. It's fairly elegant and useful, since the quest for the largest sensor regularly figures into peoples' camera-buying decision.
Oh, and if you're still not clear on that whole APS-C vs. Full Frame (vs. Four Thirds vs. APS-H, etc) then you can check out this page for our handy chart.