Actually, I use the CMG SRD pdf almost exclusively now, both in-game and out. It's ridiculous how awesome it is. I can consistently find rules quicker on my laptop than my players can looking through their books.
I'm going to put my vote in for the Hypertext SRD (www.d20srd.org); however, I haven't used the Sage SRD, so I may be a bit biased. Since I started running games online, it has been a major time saver.