You can undo
git add before commit with
git reset <file>
which will remove it from the current index (the "about to be committed" list) without changing anything else.
You can use
without any file name to unstage all due changes. This can come in handy when there are too many files to be listed one by one in a reasonable amount of time.
In old versions of Git, the above commands are equivalent to
git reset HEAD <file> and
git reset HEAD respectively, and will fail if
HEAD is undefined (because you haven't yet made any commits in your repository) or ambiguous (because you created a branch called
HEAD, which is a stupid thing that you shouldn't do). This was changed in Git 1.8.2, though, so in modern versions of Git you can use the commands above even prior to making your first commit:
"git reset" (without options or parameters) used to error out when you do not have any commits in your history, but it now gives you an empty index (to match non-existent commit you are not even on).
Documentation: git reset