One of the oldest RPG systems in existence is Chaosium‘s Basic Roleplaying System that powered games like Call of Cthulhu, RuneQuest or Stormbringer. Last year Chaosium finally released a generic BRP rulebook that covers everything from fantasy to science fiction in one book.
If you are still unsure about whether you want to buy a copy of the BRP book or not, you should have a look at the free BRP Quickstart PDF. The 48-page PDF has been released just recently and contains character creation, rules (including combat), a couple of short adventures from various genres and even some monsters.
If you ask me, the BRP Quickstart PDF is one of the best quickstart products I have seen. Especially the fact that they added several adventures is a nice touch. You can download the BRP Quickstart PDF here at the Chaosium site. Please note that you’ll have to register an account there in order to be able to download the PDF.