arith (previously known as farli, before rationalisation of my naming scheme...) is not one of the more impressive computers in the collection. This beast is a portable, in a single A4-size flat box, with a 6 line LCD screen. It's only got 32K of RAM, which is used for both temporary and long-term (battery-backed) storage! It does have a word processor plus BBC BASIC in ROM. Really, this needs an upgrade - you can move to 128K or so of internal memory, and there are newer OS versions around as well. It might be amusing to see if I can get UZI (Version 7 Unix for CP/M machines!) to run on it, but I doubt I'll have time.
I've recently found information on installing a 512K RAM instead of the rather undersized 32K - it's not technically difficult, but it does require unsoldering the existing RAM chip.
I've started taking my lecture notes (in TeX) using arith. The nice thing about the Z88 is that you can put it to sleep (turn off the display, halt the CPU) instantaneously and turn it back on again without having to save/load work. This means that you can turn it off whenever you aren't typing, to save the batteries.
Z88 Forever! is a useful site with lots of information on the machine and a reasonable collection of links to other places. The Z88 Developers' Notes are also online.
I bought arith second-hand via a posting in uk.adverts.computer, if I recall correctly.