Meld is the slickest, most intuitive, best looking different and merge tool around. It's on the Git list of tools so in your global config file set the key diff.tool to meld and it just works. It also works pretty painlessly in Windows.
You might see a warning when you call Meld from the command line, but ignore it.
One of the nicest features on Linux is VCS integration. It works with great with Git as a desktop app to view different of modified or staged files and will even commit them from Meld!
Update (2012-03-13): If you want a windows .ico file I found one here on Harvest of Gnar whatever that is. BTW: I ditched my windows batch file, and just used a shell script, which I needed for Git anyway. It has 2 lines:
python meld "$@"
Then I saved it as meld.sh, but I also made a link to it ln -s meld.sh meld, which basically copied the exact same file and saved it as meld. Finally I made a shortcut to the desktop and used the icon from above. To add it to Git just use git config --global diff.tool meld. Then git difftool to see your diffs in Meld.
Update (2012-03-23): Just learned I can drag and drop files directly into the choose files dialog! Fast and easy and way cooler than browsing to them. Now if I could just whip up a quick shell integration...
If you're a cygwin nut, see my post on building Meld for Cygwin.