For all my present Diff / Merge needs I'm using Beyond Compare; when I decided to buy a license for it I tried other similar tools, both payware and freeware.

Now BC is at version 3, and I think it's a great tool... but what are your experience in this field? Do you think there is something better? And what are the feature you like best on your favorite Diff tool?

EDIT

I'm recollecting here a list of the tools mentioned in the answers below, in order of preferences (more or less), separating pay- from free- ware and indicating supported operating system. Hope this helps.

PAYWARE

• Beyond Compare (win + linux) - http://www.scootersoftware.com/

• Araxis Merge (win + osX) - http://www.araxis.com/merge/index.html

• ExamDiff Pro (win) - http://www.prestosoft.com/edp_examdiffpro.asp

• ECMerge (win, osX, linux) - http://www.elliecomputing.com/Home/default.asp

• MergePlant (win) - http://www.mikado-ltd.com/

• Changes (OSX) http://www.changesapp.com

• Deltopia DeltaWalker (win, osx, linux) http://www.deltopia.com/

FREEWARE

• FileMerge (OSX) - http://en.wikipedia.org/wiki/Apple_Developer_Tools#FileMerge

• Tortoise SVN (win) - http://tortoisesvn.net/

• WinMerge (win) - http://winmerge.org/

• ExamDiff (win) - http://www.prestosoft.com/ps.asp?page=edp_examdiff

• Diff Merge from SourceGear - http://www.sourcegear.com/diffmerge/index.html

• Perforce Merge (win + linux + OSX) - http://www.perforce.com/perforce/products/merge.html

• meld (linux) - sudo apt-get install meld http://meld.sourceforge.net/

• Vimdiff - vim distribution

• KDiff3 - http://kdiff3.sf.net/

• ediff - EMacs distribution

• Tiny Hexer

• Kompare (KDE, linux) - http://www.caffeinated.me.uk/kompare/

• tkdiff (win, linux, osX) - http://tkdiff.sourceforge.net