MonoDevelop is the tool that comes with Unity that's used to write scripts. It's invaluable, has phenomenal auto-completion, and really I wouldn't want to code without it.
Somehow I deleted something essential to its operation and it crashes on every save. I'm trying to revive it now with every inch of my technical know how but it may be too far gone.
Edit: (My Solution)
I ended up installing the standalone MonoDevelop, which is actually a much more current version than the one packaged with Unity. It doesn't Sync with Unity quite as well, and you have to be delicate with it, but it's capable of working with a GIT repository on Windows being active in the same folder without crashing (Especially if you disable the GIT plugin), which is more than I can say about the built in Unity version (which was crashing left and right, unable to save anything).
I cannot open files from within Unity, but I can create a complete Unity MonoDevelop project by clicking "Sync MonoDevelop Project" I get all the code completion, the Assembly Browser for Unity functions, the help, the debugging... all I have to do is manually look from the Unity 3D console to the MonoDevelop line number to make everything work. And I can open files from the MonoDevelop project hierarchy. So you know all in all I'm not complaining.
It works 95% of the way, the only inconvenience is I can't directly click on Unity Console errors to have them pop up, and I have to remember to open scripts in the Mono hierarchy, not the Unity one.