Based on the criteria of sheer moddability and user-created content, Kerbal Space Program is an unmitigated success. For a game that is still considered an alpha build, there has already been a massive outpouring of user generated content. Below are some of the community's top picks.
MechJeb is a plugin that features several automation utilities that help to speed up/reduce the difficult of plotting and performing maneuvers. For example, instead of launching your rocket and controlling every aspect, you can instead program in a desired orbit altitude, turn on the autopilot, and MechJeb will do the necessary burns and staging to achieve orbit. Other functions include attitude control (e.g. maintain prograde, retrograde, node), landing guidance (from calculated landing zone to fully autonomous landing) and a docking autopilot.
While its use as a timesaver is invaluable, its usage has a somewhat polarizing effect on the KSP community. Proponents espouse its reliability and ease of use while detractors say that it makes the game too easy or that people use it as a crutch. In any case, there is no denying that MechJeb is an extremely powerful plugin with a great deal of utility.
KSP at its heart is a game about building rockets, and it does that very well. When it comes to building planes, however, the results are less than optimal. The default drag model in KSP is not very good. It doesn't model aerodynamic drag accurately, and in some cases models blunt objects as more aerodynamic than conic ones if the blunt ones weigh less. Ferram Aerospace Research (FAR) rectifies this by modeling wings and aerodynamic physics more appropriately. New concepts include stalling, mach effects, lifting body effect, and more realistic wing behaviour. It also changes the atmosphere curve to be more gradual instead of concentrating it down below 10km a la vanilla. More significantly, rockets launching blunt shaped payloads will experience more drag, requiring you to add fairings around payloads in order to maintain optimal efficiency. A must have for anyone considering building spaceplanes or other aircraft.
Kethane, which we have covered before, adds the mineable resources Ketane to the planets/moons in the game. Once mined, Kethane can be used to make fuel, oxidizer, and monopropellant, or combusted as is.
Extraplanetary Launchpads adds in new parts that allow you to build autonomous launchpads on other planets/moons. In addition, this plugin adds in a new mineable resource, ore, that can be refined into metal and rocket parts. The launchpads can construct previously saved crafts on site if they are supplied with enough metal, rocket parts, and propellant. Plop one of these on the moon and you'll be saving massive amounts of deltaV to get into obit. Perfect for really tying that colony together
This mod is gaining a lot of attention recently due to its improvement of endgame content in career mode, as well as its sheer "this is so awesome!" cool factor. Interstellar adds in fusion engines, nuclear/fusion/antimatter reactors, quantum vacuum powered plasma thrusters, an Alcubiere styled warp drive, and more. Initial techs give you basic nuclear reactors and engines that can later be upgraded to be bigger and better through the use of Science. The reworked tech-tree and increased cost requirements help to sustain career mode in the near end-game. One the defining features of Interstellar is the addition of heat/waste heat mechanics. Solar panels and reactors generate waste heat that can must be dissipated via radiators in order to prevent damage to your craft, and certain parts (such as thermal turbojets) have an effectiveness that is determined by the amount of heat available to them.
The premier parts pack for making aircraft that perform best while you sing renditions of "Danger Zone". B9 boasts some of the best spaceplane and aircraft parts around, in the form of cargobays, larger, modular wing surfaces, structural parts for building XL spaceplanes, and a variety of struts, engines, and other awesomeness. This parts pack delivers the best in performance when you just have to get onto that highway to the danger zone.
The 0.23 update brought several changes to KSP, including new engine particle effects, that rendered the SABRE engines broken. Recently, updates to the Firespitter and Exsurgent Engineering DLLs fixed these parts.
A rocket parts package, KW Rocketry introduces several new, well-rendered, and balanced rocket parts in the form of new engines, fuel tanks, and RCS systems. In addition, for those not wishing to use Procedural Dynamics (see below), it also includes its own fairing system. Welcome additions include new 3.75m and 5.0m tanks with corresponding engines, as well as new interstages and decouplers. Mostly though its about giving you the tools to make the biggest rocket you can imagine.
In a similar vein as KW Rocketry, NovaPunch 2 adds more utility and control options in addition to numerous propulsion units. New, slimmer, 1.25-5.0m reaction wheels as well as 5.0m tank + engine replica of the Saturn V are notable additions, as well as new structural panels and RCS systems.
Kerbal Attachment System (KAS) and Infernal Robotics are both geared to improving the utility and versatility of your ships.
Kerbal Attachment System adds new parts in the form of winches, piping, storage containers, and hooks. Piping can be strung between parts allowing for the transfer of resources, and winches can be used to pick up and move parts or debris around as need. One of it most notable features, however, is the ability to attach and remove parts while in orbit. Small parts, such as lights and RTGs, can be stored in cargo containers. These parts can, through the use of a Kerbalnaut, be attached to a ship while it is in flight. Say you forgot that one light that keeps your ship from being shiny. Simply place it in a cargo container, ferry into orbit, and a kerbalnaut can attach it where you want it
In a similar vein as Kerbal Attachment System, Infernal Robotics adds new parts aimed at giving more breadth of use to your crafts. These parts can be used to things such as rotating engine mounts for aircraft, telescoping landing legs, DIY cargo bay doors, and (when combined with KAS), moveable cargo winches for use in cargo bays. All these parts come integrated into a pop-up UI window in which you can group and label them, and assign different groups to different sets of hotkeys.
These three mods are all geared at improving the customization options available to your craft
Procedural Dynamics adds new, fully customizable wings to the game. By attaching the wing root to your craft you can alter the wing’s length, width, thickness, tip size/placement, and sweep to produce any desired wing shape. The plugin also adds a customizable control surface, again allowing you to fine-tune your craft to get the lift that you need.
Procedural Fairings adds interstage fairing bases and shrouds to the game. While there are several other mods out there that do the same thing (like KW Rocketry), Procedural Fairings is notable in that the fairings automatically size themselves according to the size of your payload. For example, a tall skinny payload with produce a correspondingly tall and thin fairing. If, for example, a girder is added that increases the vessels width, the fairing will resize itself according, all without your input. In addition, these fairings give you the option of generating struts inside the shell to keep your payload from wobbling around during launch. This plugin works beautifully alongside FAR as the fairings help to mitigate the effect of atmospheric drag on your ship. They also looking super cool.
StretchySRB, a continuation of the Stretchy Tanks mods, adds new fuel tanks and solid rocket boosters that can have their size and volume customized to produce any tank you need. Furthermore, these tanks can be modified to contain any type of propellant you want, as well as giving you several different textures to play around with (personal preference is the day-glo orange of space shuttle fame). In a very nice touch, propellant tanks will display their capacity, size, and weight, on the tank when you mouse over them in the VAB/SPH.
The list above represents but a tiny fraction of the thousands upon thousands of user-created mods and plugins available for this game. There are mods that add new parts, new effects, even whole new planets, all designed to enhance the game in some way and allow you to tailor the game to what you would like it to be. Even though this game is still technically an alpha build, the fact that there is already user-created content that exceeds some current titles makes it clear KSP has nowhere to go but up.