As to a tool, what you're looking for in general is something that will tighten loose screws or nuts, and maybe work to adjust a spoke if you tweak a wheel. I've never had an on-the-road chain problem, but most people think the ability to take out a chain pin can also be useful.
I suspect 90 percent of the time you can get by nicely with a small Philips-head screwdriver and folding metric Allen wrench set. Combine with a plastic tire lever and a patch kit, and you've got yourself a cheap, light, and robust setup to handle most eventualities. There are, however, good multi-tools out there as well. I like the Topeak Alien XS ($30; www.topeak.com), a light version of the 29-tool Alien. The XS model still has 17 tools, including Allen wrenches, flat-blade and Philips screwdrivers, and spoke wrenches. No chain tool, thoughfor that you need the Alien 22 ($39), which is a bit larger and heavier. Still, I have an XS in the seat bag of one of my bikes, and have found it a useful and fairly easy tool to use.
Park, maker of many excellent bike tools, takes the approach that a "single" tool is simply too hard to design. Its compact PPM-2 kit ($34; www.parktool.com) includes three stand-alone tools: a multi-ended Allen wrench and screwdriver tool, a dedicated chain tool, and a spoke wrench and tire lever set. It weighs a little more than some of the multi-tools but functions much better. It's what I have on my touring bike.