My 2 cents. Every mod brings along many changes, I personally install them one or two at a time and get used to them before installing another to make it manageable. Quite a few mods still haven't been updated for the newest patch, and there are a LOT of mods to choose from. So; Essential information; you're playing blind without this. MechJeb - It's mind bendingly enormous in what it can do, including displaying any and every single piece of information about you, your craft, and your surroundings you could ask for. It's also an autopilot in many shapes and forms. Kerbal Engineer - A slightly simplified form of MechJeb, provides essential information during construction and flight including your weight, thrust, delta-v, bearing and many, many more invaluable bits of information. I prefer it over MechJeb personally as I find it's easier to read. Does not have an autopilot, and I find flying manually is more fun. Kerbal Alarm Clock - You can set yourself alarms of any kind. Must have if you are planning on one or several interplanetary flights. Protractor - Provides invaluable information for interplanetary flights, transferring from one planet to another is painful and inefficient without it. Subassembly Manager - A mod that allows you to save specific sections of your craft. Want to switch your payload from an unmanned probe to a manned rover? You no longer have to rebuild the entire craft. Essential utilities; you're relatively limited with what you can do without these. ISAMapSat - Create detailed maps of planets and moons including height maps, allows you to plan for certain if your glorious Kerbal mun base will sit atop a plateau, or dead centre in a crater. Kethane - Introduces an entirely new concept to the game; mining for resources, in particular Kethane, a fuel you can use to refuel your crafts. Suddenly long term expeditions and bases are viable without having to endlessly ferry heavy fuel from home. Kerbal Attachment System - Hooks, connectors, anchors and more. You can now drag objects with cables, anchor yourself to ground, create utility vehicles and much, much more. How does a highly unorthodox Kerbal transportation launcher system sound? Parts; PartCatalog - Creates additional menus and tabs for your parts. Absolutely must have if you're installing several part packs and do not enjoy listing through several pages just to find that one part you want from the stock game. B9 Aerospace Pack - Introduces plenty of new plane parts, high quality and recommended. KW Rocketry - The go-to pack for more and a few bigger quality rocket parts. Includes fairings. Nova Punch 2 - Similar to Rocketry, a huge parts pack with plenty of everything. Aviation Lights - Exactly what it says. Introduces 'realistic' lights with several modes. Also good for disco parties. HooliganLabs Airships - Introduces balloons and airships. Flavour; Chatterer - Space is awfully quiet save for the roaring of engines. This mod introduces radio chatter between crafts, mission control, and remote probes. Completely overhauls the game; Ferram Aerospace Research - Introduces proper aerodynamics into the game. Nose cones and farirings have a significant purpose now. Wildly rebalances the game, and takes quite a bit of time to learn. Deadly Re-entry - Introduces proper re-entry, your crafts risk overheating and disintegrating when re-entering atmosphere. As far as I know is designed to work alongside with Ferram Aerospare Research. Makes the game much harder. and to a lesser extent, IonCross Crew Support - Kerbals actually use their lungs now; introduces life support to your craft. One more thing to manage and take care of. These are just off the top of my head, there are many more. Search around and try them for yourself.