I often have need to compare two files. Usually, I just use PSPad compare, but that tool can be quirky at best.
I tried lot of stand-alone freeware programs for that purpose, but SourceGear’s DiffMerge was one I selected. It just works (and it works fast). Along standard features for such programs, there are two features that made it special to me.
First one that I liked a lot is capability to show changes within line. It will show you exactly which characters were changed. You can quickly see what was really changed and what was just small correction. Another one is integration inside Windows Explorer. While most programs do this, this one does it properly even on 64-bit systems. That was problem for all others I tried.
If you are looking for file compare program, try this one. There are lot of features inside that I didn’t mention and program is really easy to use.