xstroke is the gesture-recognition software (analogous to Grafitti
on a Palm, and pretty intuitive for Palm users, too) bundled with
recent versions of the iPAQ Familiar distribution. It's home
page and documentation are at
http://www.east.isi.edu/projects/DSN/xstroke/index.html.
(Also, the gestures shipped by default with
xstroke are documented at
http://www.east.isi.edu/projects/DSN/xstroke/help.html). You can also configure your own gestures or change the ones
provided.