Sentilla Developer Community
It's well worth checking out. A lot of great questions are answered here: http://dev.sentilla.com/forums/
You can read about it here.
In addition to the built-in sensors that the JMotes have (accelerometer, etc) you can of course use the easily accessible ADC ports. But what sensors can you attach to an ADC port? Here are some ideas:
The Arduino Project has a list of things to try (but focus on the "Input" section of that page).
We have a few micro-radar sensors (Bumblebee) that can be attached to the JMote.
We have a couple soil moisture sensors that can be attached to a JMote, though this will need some small amount of circuitry (breadboard) plus working with the Digital Pin methods available through Sentilla's SDK.