The Membership Card is a complete computer that fits in an Altoids tin. Inspired by classic 8-bit computers like the Altair 8800 and Heathkit H8, it is thoroughly documented and easy to build, with big parts, big pads, and big traces and spaces. It uses only generic parts common in the 1980s (and still available today) -- no custom parts, and no surface mount. It's fully self-contained: You don't need PCs, Windows, megabyte compilers, or secret software to use it. Now you can learn about computers right from the ground up, and really understand how they work!
It comes as a two-board set. The Z80 Membership Card itself is a stand-alone single-board computer that can "power up" your projects, like the Parallax BASIC Stamps or Arduino microcomputers. But you need a way to program it, and of course some kind of input and output "shield" to put it to work.
Links to buy the kits are near the end of the page.