The Jornada 720 is a mini-laptop that wakes up instantly, runs all day on (new) batteries and has recent linux distributions cut to fit, replacing the long-dead Windows CE that came with it. And as Remy van Elst points out, it has one advantage over modern devices such as the iPhone: it comes with root and everything you need to run and own it as a general-purpose computer.
You can develop, compile and run native software on it. It has a C and C++ compiler, Python is available as well as Perl so it is a full fledged development workstation. Not only can you develop software on it, it can run that software as well. Here is a review of the device running standard Windows CE and here is an other review including a comparison to its predecessor, the Jornada 680.
NCommanderhas an excellent video on the 680e and the use of the device at the Dutch Railways (NS). In the past I also had a NS 680e but the battery went bad, bricking the entire unit, so I sadly had to throw that away. The Jornada 720 cost me 80 EUR on the Dutch CraigsList (marktplaats), used, but boxed including the manual and software CD-ROM.