Below you can find a list of useful references for anyone interested in modding KSP. This replaces all the old stickied threads.
Not all of the information contained below is relevant today, so please comment in this thread to let us know which ones should be removed or which ones should have a comment next to them clarifying what parts of it are still relevant. Thanks for your help!
GUIDES
Creating custom KSPedia entries (information about fonts may be incorrect)
KSP Font System - creating fonts for localization
Adding mods to the Application Launcher
Localization Support for KSPedia pages and static files
Engineer's Handbook - Developing mod parts based on real-world engineering principles
Editor Toolbar Modding Guide
Mod adoption tutorial
TOOLS
PartTools - import 3D models into KSP
REFERENCES
Modding tools, tutorials and guides
Community-driven documentation (Stock part cost and balance, contract modding, science&mods)
PartModule documentation
Library of mods that are/were in development
Module Manager patch database
LICENSING
License Selection Guide
Cross-compatibility guide to Creative Commons licenses