I've been using Beeline Reader which is basically like a Readability bookmarklet that also colors alternating lines of text subtly to make it easier to read things quickly. There's a sort of silly test on their site which showed that I read 26% faster with the Beelined text, and they claim to have performed a study that showed a 10% improvement in reading speed. I'm still skeptical, but I find it more enjoyable to read using it. Maybe I just like stuff that has "Bee" in the name, though.

Some discussion at Hacker News here.