I'm not sure what id Software, Bethesda, or Valve are thinking here but you can play all of RAGE for free until Sunday at 1PM (PT). If you already have Steam installed, all you need to do is click this link to begin downloading the game.

Yes, this apparently includes the single player component of the game. It's just odd that they would do this since most people are more than capable of downloading, playing, and beating the game in that time, thus removing the need to purchase it after this weekend. Huh, weird.
About RAGE
RAGE is a groundbreaking first-person shooter set in the not-too-distant future after an asteroid impacts Earth, leaving a ravaged world behind. You emerge into this vast wasteland to discover humanity working to rebuild itself against such forces as bandit gangs, mutants, and the Authority – an oppressive government regime that has a special interest in you in particular. Featuring intense first-person shooter action, breakneck vehicle combat, an expansive world to explore and jaw-dropping graphics powered by id’s revolutionary id Tech® 5 technology, RAGE promises an experience like no other.

Fearing a planet-wide extinction from a massive asteroid headed towards Earth, world leaders are faced with the inexorable task of ensuring human survival. Left with few alternatives, life-sustaining pods filled with select people are buried deep beneath the earth’s surface. You are one of the chosen members of these modern day arks and abruptly enter the RAGE universe after surfacing alone. Leaving you to your own devices to stay alive in a world filled with those who prefer you dead, RAGE combines powerful storytelling with heart-pounding action.