On the other hand, for less than $100 each, you can buy an almost limitless variety of very cool mass-produced 20th-century slide rules from companies like K&E, Pickett, Gilson, Dietzgen, Post (which became Teledyne), Faber-Castell, Aristo, and more. Veteran collectors tend to focus on obscure rarities, but I'd recommend just buying what you like.
Some collectors focus on private-branded models (slide rules that say "NASA," "Los Alamos National Labs," or "IBM" on them instead of the manufacturer's name). Others prefer intricate and colorful circular slide rules, or the original mahogany and celluloid slide rules of the early 20th century (later models were metal and plastic).
Whatever you do, be sure the slide rule is intact and working, since finding replacement parts for a specific model (especially the cursor, which moves up and down the device) is next to impossible.
You may already own a vintage calculator from the 1970s or '80s. People of a certain age tend to have an HP 12C, 10B, 59, 67, 45, 21, 38, or some other number hanging around in a drawer somewhere -- and a select few of those might even be worth something. An original HP-25 programmable calculator (made from 1975 to 1978) in great condition with all the trimmings will bring you about $100 on eBay. But for the most part, pockets electronic calculators are a dime a dozen.
[ Electronic calculators from the '70s are fun, but you'll get even more enjoyment crunching numbers on mechanical calculators that actually make a crunching noise. ]
The richer (and geekier) vein to explore here is mechanical calculators, whose run started in the 1800s with Charles Babbage's proposed "difference engine" and lasted through the 1960s. Mechanical calculators were produced in large volumes for business users, beginning in the 1880s with the now collectible Comptometers and Burroughs adding calculators.
But the truly educated vintage geek will want a cylindrical mechanical calculator made by Curta in the late 1940s (Type I) or mid-1950s (Type II). Designed in a WW2 concentration camp by entrepreneur Curt Herzstark and made in Liechtenstein, only about 140,000 of these compact devices were produced. They were the first truly handheld calculator, utilizing a stepped-gear calculating mechanism. Popular at the time with land surveyors and sports car rally spectators, these beauties today command close to $1,000 in good condition on eBay and elsewhere.
Of course, if you're just looking to show off a vintage calculating device without spending big bucks, I'd suggest the Little Professor calculator game, introduced in 1976 by Texas Instruments, complete with red LEDs. Instead of typing in problems, you type in the answers -- but for $25 on eBay, who cares? It's classic vintage geek.
If these suggestions have whetted your appetite, they're just a start. This is a long-tail market if there ever was one, so there are literally thousands of vintage geek items to collect and become an expert on. What's more, there's tons of detailed information on the Web for almost every one of them, so it's easy to get sucked in.
So happy shopping, and next time you're at a garage sale or flea market and see a pile of old electronics, take a closer look. There might be a treasure in there!