Those who really love to tinker might prefer an Altair. Featured on the now-famous cover of the January 1975 issue of Popular Electronics, the Altair is often cited as the machine that started the personal-computing revolution. Bill Gates and Paul Allen worked with Altair before they launched their Albuquerque startup company Microsoft. Briel Computers is taking preorders for its AltairPC, a PC case that makes a typical desktop computer look just like a 1975 MITS Altair 8800. Collector Rich Cini also makes Altair-style PC covers, but Cini's is a front-panel cover with a plug-in board that connects the panel to your PC. Download the free Altair32 emulator software, and you can explore the original functionality. A full-scale, fully operational version of the Altair will soon be available through Grant Stockly. That kit isn't officially priced yet, but it won't surpass $1,700, Stockly says.

For something a little more exotic, you could give someone a fully electronic $235 Enigma-E or a $10 Paper Enigma, the latter being well suited for children. Both can make and break codes with historical accuracy. Also good for children is the Digi-Comp I v2.0, which costs just $55. It's an amazing computational trainer that anyone can build in an afternoon.

Tore Bekkedal, a collector from Oslo, Norway, says the various replicas are good news for common hobbyists like him. "They replicate not only the fascinating technology, but also the tinker spirit," he says. "It also helps preserve the software and the knowledge of how to work it."