What tools on a multi-tool do I actually need?
Minimally, you want to be able to tighten all of the bolts on your bike (likely a few hex keys will do this) and an appropriate screwdriver for adjusting derailer &amp; brake pulls.  Separate from a multi-tool, a pair of tire levers are the other tool you should carry with you.

I would add a chain tool to the above list after being left in a state where I would have been much happier were I so equipped.  For that reason, I bought a Crank Brothers Multi Tool 17 which features the requisite hex keys, screwdrivers, and a chain tool, in package that is smaller &amp; lighter than most Leatherman tools.  Some of the hex key sizes don't fit anything on my bike, but it's nice to have a set that can help most other riders.