I know many chefs and cooks who keep a cake-tester tucked into pen-pocket on their whites and none who use them to test cakes. Not that you can't test a cake's doneness with them, it's just why would you want to when there are so many more interesting assisted-poking tasks at which it excels?
Essentially a heavy-gauge wire with a handle, it's about as simple as tools can get. The idea is that you poke it into the center of a cake and pull it out. If it comes out clean, the cake is done, sort of like a glorified toothpick. But the fact that it's long and made of metal means that it's useful for all kinds of other things.
"With a cake tester, you can find out without leaving behind any incriminating evidence of your intrusions."
The most obvious is testing the doneness of other vegetables. Have you ever been told to stick a paring knife into a boiling potato to check if its tender all the way through? The problem is that even the thinnest of paring knives makes a large stab-wound in the potato, releasing starch, and vastly increasing the chances that it'll break apart, particularly if you've bucked up for those tiny, tasty fingerlings. A cake tester neatly takes care of that problem. Want to know if those simmering carrots are tender enough to puree? How about if those baby radishes are cooked through? With a cake tester, you can find out without leaving behind any incriminating evidence of your intrusions. My favorite way to cook beets is in a tightly sealed foil pouch—a cooking method that absolutely prevents you from poking with a paring knife. A knife makes a hole in the foil too large to ever recover from. Not so with a cake tester.
I use my cake tester instead of a fork to decide whether or not my braising brisket or short ribs are "fork tender." If the cake tester slides in and out with ease, it's ready. Lots of fish have membranes between layers of flesh that soften at around 135°F or so (a perfect medium rare). Stick your cake tester through that poaching salmon fillet and it if meets resistance (it'll feel like punching through pieces of paper), you're still undercooked. Barbecuing a pork shoulder low and slow? You can check if it's done without dropping any juices through the grill grates.
Finally, if you ever (god forbid!) find yourself without your trusty thermometer by your side, a metal cake tester is the next best thing. Stick it into the center of your meat, let it rest for about 5 seconds, pull it out, and place it under your lower lip (it's an area particularly sensitive to heat). You'll instantly know whether your steak is cold, warm, or hot in the center. As accurate as a thermometer? No. Good in a pinch? You bet.
You can go all out and pay the $5 for a Cake Tester from OXO, which has a grippy black handle, but you may risk being made fun of for being too fancy-pants. The Cake Tester from Fox Run ($1.29) is the cheapest I've found online, though the nameless model pictured at the top is the one you find most often in restaurant kitchens, and one that I picked up for under a buck in Boston's Chinatown.