I've noticed a few commercial products have released free merge tools recently. I'll come onto them after the good open-source ones.
I am a massive fan of KDiff3 which provides a very good 3-way merge and has saved my merging hide more times than I can remember.
WinMerge is definitely the slickest merge tool for Windows, unfortunately it is only a two-way merge.
Meld is an excellent system but although it is cross-platform it really excels on GNOME. The comparison looks great with the two sides connecting up.
And now some commercial ones...
SourceGear DiffMerge 3.0 was released recently. Looks OK as well and it is cross-platform.
The Perforce Merge Tool was recently released for free. It looks really quite good and powerful.