How do I view 'git diff' output with my preferred diff tool/ viewer?

Question

When I type git diff, I want to view the output with my visual diff tool of choice (SourceGear \"diffmerge\" on Windows). How do I configure git to do this?

Answer 1

A short summary of the above great answers:

git difftool --tool-help
git config --global diff.tool 
git config --global --add difftool.prompt false

Then use it by typing (optionally specifying file name as well):

git difftool