They're possibly the most popular personal organisers, as they have an extremely flexible OS and nifty handwriting recognition system.
Also by TRG, Handspring, Sony and other. Try looking at PalmOS.
For "teenagers," there is [Cybiko] wireless, 900MHz peer-to-peer HandHeld. They're pretty nifty. You can talk to other people within a 300 foot radius. It vibrates too.