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