One of the very important tools for all programmers is the Diff and Merge, there are many software products out there for diff and merge, here is a useful list of them: http://en.wikipedia.org/wiki/Comparison_of_file_comparison_tools

I personally used Araxis Merge http://www.araxis.com/merge/ for long time before, and I found it very useful, but its standard license is for 129$ (you can download trail version for 30 days)

I also tried the Beyond compare http://www.scootersoftware.com/index.php and I found it even better than Araxis Merge and its standard license is for 30$

also for for free tools I am trying SourceGear DiffMerge http://www.sourcegear.com/diffmerge/index.html and I think it is good too.

Tell me, what is your favorite diff & merge tool?