JLiME, short for Jornada Linux Mobility Edition, is a Linux version for HP's Jornada PDA product family. Currently the OS supports Jornada 620, 660, 680, 690, 710, 720 and 728 models, and also NEC MobilePro 900c. Support for HP Jornada 540 is making progress as we speak. JLiME gets developed with a steady pace, and that's why it is a lot better alternative to the Windows CE operating systems which are preinstalled on the devices. JLiME is safer, and with it the user can take advantage of the many properties of current Linux systems. And besides, JLiME supports almost all available hardware for the device (e.g. memory cards and WiFi cards). Even if a hardware part would lack support, the JLiME development team seizes to it rather quickly after the user has made a topic of the issue.
The operating system is installed onto the Compact Flash card, where it gets loaded and run from. HP Jornada devices don't have the option to get JLiME installed onto the ROM drive so that the machine would load JLiME straight after the ON button has been pressed because the ROM drive is read-only. Yet the NEC MobilePro 900c can be flashed with JLiME, but the development team hasn't published instructions or files for this yet because they want to make sure everything works.
In my HP Jornada 690 I'm using Linux kernel 2.6.25, and a desktop environment called JLiME Winter (link), which can be extended in themes really easily and fast. JLiME Winter includes an image viewer program which shows an image file all the time (the theme image, or background). The image file has the buttons embedded into it, which means that the user has to paste them into the image if he decides to make a theme himself. The link I referred earlier contains five themes made by me. Below there are template images of buttons, text and icons which help in making new themes.