reset

To revert an entire commit.

git checkout <file>
re-checkout <file>, overwriting any local changes

Should be able to git add changed files that you are interested in committing the revised changes and git commit --amend

http://stackoverflow.com/questions/927358/git-undo-last-commit

git diff of files that have been staged ie 'git add'ed
git diff --cached

http://www.commandlinefu.com/commands/view/1242/git-diff-of-files-that-have-been-staged-ie-git-added

$ git commit ...
$ git reset --soft HEAD^ (1)
$ edit (2)

And just commit again.