They're not too old actually, they first popped up around 2000. The one I have is one of the first. They made several models over the following years, up to a wireless one even (I'll attach a picture), all of which looking a bit slicker and more modern than the one I have (though I like the blockier look myself). The newest model is incompatible with the code hackery I've been doing though. They completely changed processors. The older ones use a Z80, which is what was used in lots of old game consoles (like Coleco Vision), home computers (like TRS-80 and MSX), arcade games (like Pac Man, Galaga, etc), in the GameBoy (until GBA), and even still used as recent as in those fancy TI graphing calculators and such. The chip is kind of a cousin to what was used in PC's.
Anyhoo, you used it to send and receive email without needing a PC, by hooking a phone line into it. Their profits came from you paying a monthly fee to use their email service through it, though clever folk knew ways to get around that. In the end, I guess they realized people tend to actually use PCs for email, and they stopped making'em. Cidco made them to start with, then it changed hands to Earthlink at some point or something.