The Git documentation lists some fairly common aliases:
git config --global alias.co checkout
git config --global alias.br branch
git config --global alias.ci commit
git config --global alias.st status
Here are some of my personal ones:
# overwrite previous commit message
git config --global alias.amend commit --amend -m
# unstage files
git config --global alias.unstage restore --staged
# delete a tag, both, locally and remote
git config --global alias.untag '!git tag -d $1 && git push origin :refs/tags/$1'
# if git is locked, e.g. due to an aborted pre-commit hook
git config --global alias.unlock '!rm --verbose ./.git/index.lock'
# set upstream when git pull/push fails
git config --global alias.sup '!git branch --set-upstream-to=origin/$1'