If you'd like to delete all local branches that are already merged in to the branch that you are currently on, then I've come up with a safe command to do so, based on earlier answers:
git branch --merged | grep -v \* | grep -v '^\s*master$' | xargs -t -n 1 git branch -d
This command will not affect your current branch or your master branch. It will also tell you what it's doing before it does it, using the -t flag of xargs.