There are older mods that, when introduced, created quite a furore in the community. MechJeb is certainly one of those. In-game, it's a mechanical copy of our hero, Jebidiah Kerman's, neural patterns ...
Greg Laabs, better known to Chatty as OverloadUT, is one of the community's most dedicated Kerbal Space Program players. So there was no better authority to turn to for a guide to game's numerous mods ...