MechJeb - For info screens and because I find manually launching to LKO tedious. KAC - A must for running multiple missions at once RCS Build Aid - I like my ships to be stable KIS/KAS
Final Frontier & Kerbal Construction Time - These 2 make the game more fun and engaging for me.
Janitors Closet - The newest addition to my list. Just started using this one a few weeks ago and it really makes it easier to find parts in the editor quickly.
Plus many parts packs.