Agreed with interactive rebase, but what of reflog? I've checked that for commit hashes when things have gone very wrong but I don't know many commands.
It allows you to observe and interact with the history of refs like HEAD, so superficially destructive operations like amending a commit can be recovered, reverted, etc. It's kind of like meta-git, allowing version control operations on the version control system. It's a lifesaver when you botch a merge or something and allows you to do "destructive" operations fearlessly (up to a point, you can munge a git repo pretty bad if you try hard enough).
