Some time ago Sameer Verma from the OLPC San Francisco community posted a video of a simple LEGO WeDo robot which he programmed with Scratch on an XO. Now more recently he published yet another great video where he measured the voltage from a Zinc and Copper lemon battery using the Measure activity:

After Guzmán Trinidad's pulse meter this is the second very interesting application of an XO to measure an environmental variable I've seen in a short time. Now I definitely can't wait to see what Sameer, Guzmán, and others come up with next!