Generic, Endlessly Applicable Roleplay System (3rd Ed.)
This is the homebrew system that I use for a lot of the games that I run, including The Chronicles of Caledain, my current on-going fantasy campaign.
It is a rules-light, skilled-based system, requiring only a d20 and (occasionally) 2 d10s.
Being fairly simple and light, it’s extremely adaptable to any genre, and to date I have used it (or previous versions of it) to run games in the following genres: Swashbuckling, Steampunk, Horror, Fantasy, Cyberpunk, Super Heroes and Post-Apocalyptic. I currently have plans for Arthurian and Space Opera games as well.