Mass Checksumming tool for Windows?
hashutils and HashCheck are the best packages I've found on Windows so far. Both tools are open source from the same author, but HashCheck is a shell extension and hashutils is a set of CLI tools. In my testing, they're significantly faster than the ported unxutils versions. (Despite the expectation that they'd be I/O bound.)
I also think that these tools are better than the often-mentioned HashTab, and if I could get to the Ars OpenForum, I'd link a post I wrote up with my justification.