The AlphaSmart Dana has a similar form factor to the TRS-80 Model 100. It runs Palm OS and can easily be converted to run off of standard rechargeable AA batteries. The downside is that the batteries only last for 20-25 hours of screen on time.
Those are getting hard to find. There are still new similar machines being made.
The Pomera DM250 is one that looks pretty interesting to me. Occasionally I check for any progress on hacking the thing to run other software but I haven't found much so far.
