Jed (the source spreadsheet maintainer) does a really good job of keeping the newer companies up to date with public info, but it's hard to find a lot of public info on YC companies before the fact that they were YC was newsworthy.
This list is a good start, but far from up to date; just see, this (http://www.google.com/search?num=100&hl=en&safe=off&...) for YC 10.
And I would absolutely appreciate any/all corrections. I know the early years of YC data aren't as accurate as the last few.