Git bisect is by far my favourite git tool. It’s not that I get to use it that often because it is only used for finding bugs and the like. Yet, when you need it it is extremely valuable. Git bisect lets you do a binary search (a bisecting method) on the git commit history.
Imagine you are working on a project and would like to include someone else’s work. Luckily, that work is already on Github. Submodule helps you with that.