RuneQuest 6 ruleset, but the price of the PDF was never one of those things - $25 is steep, no matter hoe you cut it.
$5 is cheap, way cheap for what you get, and it's this price for 1 day only. If you are at all interested in RuneQuest, this is you chance to check it out. Do it.
There are other rulesets that are one sale for 4.99 for 1 day only:
Broken Shield - Superhuman Sci-Fi Noir Roleplaying
HELLAS: Worlds of Sun and Stone 2nd Edition
Midgard Campaign Setting - from Kobold Press
There's a lot of stuff on sale at RPGNow at the moment. Let me dig through and present some more highlights later ;)