Nutdrivers are tools usually associated with the electronics industry, but they are also very useful for in-shop bicycle work. A nutdriver (also known as a "spintite") consists of a socket wrench, generally six-sided, with a screwdriver-type handle. It usually has a hollow shaft, so that it can be used to run a nut a long way onto a bolt, with the excess length of the bolt going up inside the shaft.
For bicycle work, I use three sizes of nutdriver -- 8, 9, and 10 millimeters. In these rather small sizes, the screwdriver-type handles are speedy to use, but you can still apply enough torque to tighten the corresponding nut and bolt sizes securely. Because of the limited amount of torque provided by these handles, however, you would have to be a real gorilla to overtighten anything with them. This is a useful safety feature.
A real timesaver in adjusting the most common types of caliper brakes is to use a 10-millimeter nutdriver to tighten the acorn nuts that hold the brake shoes to the brake arms. In the same operation, you can use the nutdriver to bend (straighten) the brake arms for good parallel contact between the brake shoes and the rim. [No longer recommended, due to initiating fatigue failure of aluminum alloy brake arms. Use brake shoes that allow angle adjustment. Many use now use Allen wrenches instead-- John Allen] A nutdriver is also the best tool for tightening brake-cable anchor bolts, especially on centerpull brakes.
The most well-known and widely distributed brands of nutdrivers are Xcelite and Vaco. Both are first-quality tools. Unfortunately, it is not always easy to find them in metric sizes. (To substitute for the 8-millimeter size, the readily available 5/16-inch size is quite a good fit in practice.) Metric sizes can be specially ordered by any quality electronics supply store, however, and they are also readily available by mail order, usually at a somewhat higher price. One mail-order source is Jensen Tools of 1230 Priest Drive, Tempe, Arizona 85281. Their catalog is a treat for anyone who appreciates fine professional tools. [That information is obsolete! An Internet search will find many suppliers; for example, Tool Source -- John Allen]
Xcelite also makes a modular "Series 99" system with interchangeable blades and handles. [An online source for Xcelite Series 99 ] I have seen a number of similar systems, but none with the same quality and versatility. In addition to nut driver blades, the company has all kinds and sizes of screwdrivers, Allen drivers, Bristol drivers, Pozi-Driv drivers, and you name it! There are several sizes of screwdriver-type handles and "T" handles, with and without ratchets, a couple of sizes of reamers, and extensions. (The extensions, used with a conventional screwdriver blade, make it possible to tighten loose Zéfal HP pump heads, which are held by a screw that is only accessible from the inside of the barrel!) I have been using this series of tools for several years with satisfaction, except that the ratchet handles, while a joy to use, don't seem to be durable enough for heavy-duty use.
A variation on the nutdriver is the "T" wrench. The best "T" wrench for bicycle use is the Campagnolo. [No longer made -- John Allen] This has an 8-millimeter head like that of a nutdriver, but instead of a screwdriver handle, it has a crossbar at the end of the shaft, making the tool look like the letter "t" This crossbar gives more leverage than a screwdriver handle, but since the handle is thin and has sharp corners, you can't comfortably apply much more torque than with a good screwdriver handle. The real advantage of the Campy "T" wrench is that the crossbar is also a very useful 6-millimeter Allen wrench. If you have an all-Italian-equipped bicycle, this one tool will fit almost everything - and it is a very light, compact tool, good for on-the-road repairs. The 8-millimeter socket fits almost everything on Campagnolo brakes and derailleurs, the dropout bolts that hold on racks and mudguards, and the bolts that hold the toe clips to the pedals. The 6-millimeter Allen wrench fits the rear derailer, the expander bolt of most Allen key handlebar stems, and some seatpost bolts.
Another variation on the nutdriver is the "Y" wrench. This is a three-ended socket wrench in the shape of the letter "Y" It is generally available in bicycle shops in two sizes - 8, 9, and 10 millimeters; or 12, 14, and 15 millimeters. The 8-9-10 is a very versatile tool to carry for on-the-road repairs, as it is very small and light. It is also okay for shop use, although not quite as fast to use as true nutdrivers. I don't particularly recommend the 12-14-15 "Y" wrenches. They are too bulky and heavy to carry in a road tool kit, and I find them too awkward for efficient shop use, although I know several excellent mechanics who disagree with me on this point.
Neither the "Y" wrench nor the "T" wrench has the hollow handle of a true nutdriver, so a bolt can bottom out in the socket if the bolt projects more than a little through the nut. This is not usually a problem with routine bicycle work, but can cause trouble in some instances, especially when installing luggage racks, bottle cages, and the like.