This is completely off-topic, but what is the deal with grey font colours on light backgrounds these days? It pointlessly makes it harder to read. I shouldn't have to open Firebug to change it to black before I start reading.
I've got the same problem and gripe.
Apparently one cause is design mock-ups in which text is ipsum-loremed and muted to reduce distraction from the overall design.
Other useful tools include console-mode browsers, or plugins such as Readability Redux. Though I find my self increasingly editing out elements (in addition to fonts, especially static headers/footers) which get in my way.
But the back ground shouldn't be darker than #cccccc
I think, but I do not know, that some people with Aspergers prefer a lower contrast. (See, for example, early websites using silver background with black text.)
There was some research in the 1970s or 1980s about maximum readability for paper texts which arrived at similar conclusions. A dark brown text on matte cream paper tested best. I've read several books printed in this format, and found it quite readable. Though paper still beats screen for readability (but not grepping).
What gets me are the sites who somehow break the generally very good scaling that Safari lets me do to make the text bigger.