I wrote my own for Trackbacks, which I used to incorporate trackbacks into http://on10.net, but Geoff provides one on his blog (that doesn't appear to support comments, but does support trackbacks, so I thought I had better blog about it to thank him) that does both.
Silly me, I hadn't built pingback support into on10.net, thinking that Trackbacks would handle most cases... but it turns out that many sites (including wordpress) only send pingbacks... oh well, at least it will be there after our next sprint.