Numbers
The ACLU cites "strong evidence" on their website that seven people have been executed since the 1970s even though they were "probably innocent." So far, I haven't located more information.
A Stanford Law Review study (I haven't found the full paper online yet, but the reference is "Miscarriages of Justice in Potentially Capital Cases," 40, 1 Stanford Law Review, 11/87 by Bedau and Radelet). estimates that out of about 7,800 executions since 1900, 23 were executed that the authors believe were innocent. From analysis I've seen by others, the authors have no certain proof of this, but to be fair, they don't claim such proof either.
Since I have no link so far to original materials, you can take or leave this information.