Working in multiple repositories with multiple branches and many PRs, it's very easy to lose track of what has been merged. That's why I'm glad I've found this command.
# Shows a list of branches that have been merged to master
git branch --merged master
# Combine it with grep and automatically delete all merged branches.
# Just make sure you're on master when you do that.
git branch --merged master | grep --invert-match '^\*' | xargs -n 1 git branch -d