My Zire 21 was something I bought for my father as a beginner's, entry level PDA about 5 years ago. When he died in 2005, I ended up inheriting it, and it's been extremely useful.
Let's not beat around the bush here; this is about as basic a Palm as you will get nowadays. 8Mb of onboard memory, black and white screen (or technically, black and irritating greenish-grey screen), no backlight. It does everything the flashier, full colour palms do, however. Weighing in at about the size and weight of a pack of cigarettes, it is terribly portable, and has a rugged design which survives a lot of knocks; I happened to have it with me when I was forced to walk home through 15 miles of torrential flooding, sometimes waste deep in water, and it survived where my MP3 player didn't.
The major selling point is the price: Five years ago, I bought it new for 40GBP. Nowadays, it sells for even less. If you're wanting an electronic organiser for peanuts, you really can't do better than this.