Apple - Pro - Tip of the Week - Speed Tip: Faster Full-Name Viewing in List View.
Arggghhh. This has annoyed me for years. Now I discover there's a solution. In columnar view, to see a complete file name, hold the option key and place the mouse cursor over the truncated name (don't click).
I'm going to bang my head on the desk now.
Update: Another way to do something similar is to invoke Cmd-Opt-I. The resulting information display window changes as you click on an object. If you display the name field you can even edit file names this way. It's surprisingly convenient, especially in icon view. BTW, if you select several items and do Cmd-I you get an information window for each one, but if you select Cmd-Opt-I you get a summary window for all three. So clever. In the OS X world, it hardly ever hurts to try the Option key and see what happens. If the Cmd key is the "splat" key, the Option key is the "easter egg" key.