It's been my experience that .21 has really messed things up as far as mods are concerned. I've run all of the following mods in .21 to some degree of success, just not all at the same time. . . Chatterer - Just gives the game that authentic feel. MechJeb2 - With the new ASAS I barely use it, but it helps if you're new to the game. Aviation Lights - Again just adds a little touch of realism and authenticity. NovaPunch - I just like having all of the extra engines and rocket parts. Firespitter - Essential if you want to build rotor and prop driven crafts. Soviet Pack - Like the historic rockets available in this pack. B9 Aerospace - Amazing pack from BAC9, I just can't seem to run it with too many other mods installed in .21. Lazor System - I run it with most of the additional parts sets - Missiles, Docking Cam, ect... Crew Manifest - It's definitely not obsolete, yet, but most likely soon. Anvil Rockets - Cool pack, but the 3D models need a lot of work. It has some seriously heavy lifting rockets for those real large payloads. Mods I am sadly missing since the .21 update. . . ISA Mapper - Without this mod, building probes to other planets is meaningless. Damned Robotics - I don't know is this the alternative mod out for .21? FASA Mercury & Gemini Rockets - Missing this one so much, I dig the two man Gemini capsule. KW Rocketry - Can't wait for this to be updated.