Git
Decentralized version control system.
Searching for when a given string was introduced
When you want to find out which commit first referenced a given string:
git log -S <string to search for> --source --all
See this stackoverflow answer.
Reverting commits without creating a new one
This is useful when you want to revert a set of commits, but also when you want to change them before committing again.
git revert -n <commit hashes to revert>