I've been downloading lots of free RPG stuff recently and I've come across all kinds of wonderful stuff. Some of it is quickstart rules, some of it complete rules systems, some of it just fun stuff you can get some use out of, maybe even a campaign.
So, below are a few links where you can find some cool free downloads. (If the link doesn't work then apologies, but it's no doubt been changed since I posted this.
I've started this with Darkshire's list of links to free games in general - this is a fantastic list and there's some great games on here.
Then there's Chaosium's Call of Cthulhu Quickstart stuff.
Oh, and their Basic Roleplaying Quickstart.
Rob Lang's ICAR is always a winner.
AntiPaladin Games Minisix - a cutdown version of the D6 system - is excellent.
MicroLite20 is a great game for all you D&Ders out there who want things less complicated.
The Dragon Warriors Introductory game is always worth a look. (This link takes you straight to the PDF).
Savage Worlds is a good 'un.
A Song of Ice and Fire has a quickstart - and as I'm enjoying Game of Thrones right now, this is a good one.
An excellent Deathwatch Introductory Game from Fantasy Flight Games is here.
And I think I'll leave it at that for now. Some of these games you can get a half-decent campaign out of, but nothing quite takes the place of the full game, with all the options and opportunities it gives.