B9 AerospaceThis pack is the single best addon to come out for KSP in a very long time. It includes a wide variety of parts for building spaceplanes and other advanced spacecraft as well as jets, VTOLs, and cargo planes. The modelling is exquisite and the attention to detail is impecable. B9 Aerospace is a must.
KerbXKerbX is a Kerbal analogy of SpaceX's series of Falcon rockets and the Dragon capsule. When used as a complete package these rockets provide a sure-fire way to get your payloads to orbit. The current release includes the Falcon 1, Falcon 9 and Dragon.
KW RocketryKW Rocketry is a comprehensive launch vehicle construction pack containing tanks and engines of varying sizes from 1.25m to 3.75m. KW Rocketry has recently been updated to work flawlessly with KSP 0.20
Kosmos Space Station PartsThis pack is one of the most comprehensive collections of parts for building spacecraft and stations. It is modelled off the Soviet TKS spacecraft and Salyut Stations and has been expanded to include some ISS-size solar arrays and other useful parts. Includes VA Capsule, TKS, Salyut, APAS 69, and Docking Nodes.
MechJeb 2MechJeb is a flight assistant autopilot by Anatid Robotics and Multiversal Mechatronics. It was developed after analysing the brain of Jebediah Kerman to see what made him such a fantastic pilot. Mechjeb can pretty much do anything when it comes to flying your spacecraft from ascent to landing.
MechJeb 2 has been completely rewritten to take advantage of the manoeuvre node system that is now available in KSP. Thanks to this it's now even more useful than ever allowing you to use it for complete automation or just automatically planning your manoeuvres so can do the burns yourself. Even if you don't use it to fly your ship, MechJeb's useful readouts can be invaluable.
TouhouTorpedo's Pitch Vector EngineThe TT Pitch Vector Engine has revolutionised the way I launch spaceplanes because it allows you to offset the poor balance of a side-mounted shuttle. Just like the real space shuttle you'll be launching your spaceplanes just like the real shuttle once you've got the hang of the controls.
Quantum StrutsIf like me you were getting tired of the spacecraft you assemble in orbit being all wonky and exploding at the drop of hat, Quantum Struts are for you. These struts can be toggled to create strong joins between docked spacecraft for a minimal cost of electrical energy.
There's plenty of other great addons for Kerbal Space Program on Kerbal SpacePort and the KSP Forums. Also keep an eye out for our upcoming addon spotlights for RemoteTech.