iPAQ-under-Linux bookmarks...

General documentation

• Michel Stempin has a Linux iPAQ HOWTO Homepage at http://mstempin.free.fr/linux-ipaq/. This is mainly a guide to installing Linux on the iPAQ, and it isn't terribly general yet (for instance, it assumes USB networking).
• There are a number of other FAQs and HOWTOs linked at http://www.handhelds.org/handhelds-faq/handhelds-faq.html.
• There's a Wiki (set of community-maintained documentation) at http://www.handhelds.org/z/wiki/HandheldsWiki; once you have Linux on your iPAQ, that's a good place to browse to find out what else you can do.

System software

• `Familiar' is the Linux distribution I (and most iPAQ Linux users) use. It's inspired by Debian, but much stripped down. Its home page is http://familiar.handhelds.org/.
• 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.

Development

• HP maintains a cluster of iPAQs (and similar platforms) for development. Nick Duffek (who manages them) has notes about how they're set up at http://handhelds.org/projects/ipaqsetup.html. You can use that as a guide for setting up your own iPAQ (with additional disk space in some form) as a development environment.

Other software

• Richard Cohen has made the ogg123 command-line tool for playing Ogg Vorbis audio available (along with libraries it depends on) at http://ipaq.vmlinuz.org/ogg/. This is all based on Nicolas and Philip Blundell's conversion of the Vorbis codec to integer arithmetic. (The reference implementation from xiph.org is floating-point, and since the iPAQ's ARM processor doesn't have hardware floating-point support, the reference implementation is far too slow to play oggs in real-time.)
• Sam Engstrom has some sketching tools and a build of Dillo, among other things, at http://www.samengstrom.com/ipaq/.

Hardware and peripherals

• At http://www.gomadic.com/ipaqipaq.htm, Gomadic sells the iPAQ end of an RS232 connector cable (for about $10) - i.e., it's the connector that plugs into the bottom of an iPAQ, attached to a short cable, at the end of which are exposed wires. Not all the contacts are wired; you can use this to make an RS232 cable, but you can't use it to make a USB cable. I suspect you probably couldn't make a charger with it, either, although I'm not completely sure of that. They also sell a charging USB sync cable (for about $15) and a raw iPAQ connector plug assembly (for $2.50) which you could wire to make a USB cable, a serial cable, a charger cable, or all three at once (if your soldering hand is steady enough).
• On a similar note, pocketnow.com has a two-part series on wiring serial connectors to connect various things to your iPAQ. Here are the first part and the second part.