So I use WinMerge when I compare sets of files to each other. It's free, it's open-source, and I've found it works remarkably well.
http://winmerge.org/
What do you guys use? Or do you not find the need to compare files?
I use WinMerg as well, but I find that even after tweaking the default settings (IE finding groups of text that have changed order) it still isn't as good as some commercial apps.
I use WinMerge and to comparison tool in Subversion (thru TortoiseSVN).