This page discusses existing mods that are for the latest version (Version 1.2)only. If you want to learn how to make mods, see Modding Tutorials.
Note: This page does not, and never will, contain the thousands of available up-to-date mods.
A somewhat complete list can be found at the Mods->Released section on the forum and on the Steam Workshop. Each of these will have exclusive mods as well.
Mods can be acquired using the steam workshop page (if using steam version), through Ludeon Forums, or through third party websites (use at your own risk!). When using the steam workshop, all you have to do is to subscribe to a mod, and steam will do the rest. It takes more effort to install mods from other places, however. This is done by downloading mods, then moving the mod files to the "mods" folder in the game files. The game files are accessed through typing "appdata" in the search bar of the file explorer, going to the LocalLow folder, then opening the "Rimworld by Ludeon Studios" folder.