Given an eight-character CUSIP, this tool computes the check digit and provides the full nine-character CUSIP.
This tool was tested against the example CUSIP list here: 126650BG4, 254709AC2, 437076AQ5, 441060AG5,50075NAN4, 574599BE5, 617446B99, 637640AC7, 713291AL6, 852061AE0, 887317AA3, 925524BF6, 125509BG3, 125896AV2.
It is based on the description of the algorithm given on Wikipedia.