Grim Tales is a self-contained* ruleset by Badaxe Games.
It's basically D20 Anything Not Too Specific. Along with a simple core set of rules a lot like D20 Modern, it has optional rules for magic, firearms, vehicles, horror/insanity, cybernetic implants, and so on. You can build almost any sort of campaign with GT, excepting of course really specific settings like Star Wars or FR, which are especially specific about the way things work. For example, you could pretty easily do an FR game with GT, but not if you went into the specifics about the culture surrounding specific classes, stuff about how magic works, etc.
Anyway, I use it for a fantasy** setting, am going to be playing with it in a mostly-historical 17th century pirate game that starts tomorrow, and am planning a new dystopian Half-Life 2 inspired setting with a friend, which will be using GT as well.
*Technically requires you to either have D&D3.5 or D20 modern, but you can use it just fine if you already know the basics about character creation in d20 games.
**Fantasy in this context: Pseudo-earthican world with rare and dangerous but powerful magic, realistic cultures (yes, I can use the 'R' word here, because pretty much everybody is human, the geography is comparable, and magic/monsters/gods/etc. aren't common/important enough to change the nature of human culture), and some strange monsters--like maybe one dragon in a whole campaign, a couple of evil spirits, etc.--that are so far divided from most people that they might as well be mythological. Oh, and the regions focused on--which have the world's highest levels of tech--are at the same level as 13th-14th century europe, with the same overall aesthetic.