Aside from the standard stuff I have:

  # Usage: git whatsnew <other_branch>
  # Show commits that are in the current branch,
  # but not in <other_branch>. Great for an
  # instant changelog.
  whatsnew = !sh -c 'git shortlog --format=\"%h %s\" $1..HEAD' -

  # Usage: git omgwtfbbq
  # abort/reset/clean/etc everything back to HEAD.
  # DESTRUCTIVE!  Handy if you get git into a
  # really odd state, during a merge, though...
  omgwtfbbq = !sh -c '~/.bin/git-omgwtfbbq'
 
  # and in ~/.bin/git-omgwtfbbq
    #!/bin/bash

    # Get confirmation from user
    read -p "This will erase any work done and reset to HEAD. Continue? [yN] " -n1
    if [[ ! $REPLY =~ ^[Yy]$ ]]
    then
      exit 1
    fi

    echo ''
    # Reset everything
    git clean -f && (git rebase --abort || git reset --hard)

[edit: formatting]