The Mitchell Report

Available here (pdf). It's lengthy; if you skip to the end of the report and backtrack, you'll find the appendix that includes all of the hard evidence: personal checks and the like.

According to McNamee, from the time that McNamee injected Clemens with Winstrol through the end of the 1998 season, Clemens’s performance showed remarkable improvement. During this period of improved performance, Clemens told McNamee that the steroids “had a pretty good effect” on him. McNamee said that Clemens also was training harder and dieting better during this time.

Clemens apparently became interested in using steroids after a road trip to Florida in early June. According to the report, he began injecting "later that summer," and, as noted in the above excerpt, saw significant improvement. There aren't any specifics as to when he started, but it's interesting to look at his first half/second half splits:

First Half: 3.55 ERA, 9.1 K/9IP, 4.2 BB/9IP
Second Half: 1.71 ERA, 11.7 K/9IP, 2.5 BB/9IP

Of course, he was also "training harder and dieting better"; I'm not suggesting this is some sort of smoking gun in regards to the impact of performance enhancers.

I'm glad to see that the Big Hurt isn't among those incriminated, not that it would have changed the way I feel about the guy. Quoting the report again:

Among current players I asked to interview were five who have spoken publicly about the issue. When I did so, I made clear that there was no suggestion that any of the five had used performance enhancing substances, and I repeat here that clarifying statement. Four of the five declined. One of them, Frank Thomas of the Toronto Blue Jays, agreed. His comments were informative and helpful.

[Update: Deadspin has compiled a list of the players named in the report.]