I'm currently wearing a F-91W with one of these installed. Here's what I have running:
* Clock
* Sunrise and Sunset times
* Die roller (d2, 4, 6, 8, 10, 12, 20, and 100)
* Stopwatch
* Temperature sensor
* 36-hour temperature log
The die roller was the main one that I was excited about, but the sunrise/sunset seem useful while hiking or backpacking, and the temperature sensor and log are great for camping use. Great job by @josecastillo all around!