HaloScan has implemented a Trackback service with their commenting service. As much as people complain about HaloScan commenting, they've really done a great job supporting their user base; they respond quickly to bugs or problems and they really do provide a great service for the price.
Um...that would be free!
Actually, I should say, considering the price, they provide an amazing service. If you know how to use Trackback, go to it! If not, please visit the HaloScan site, they explain it much better than I ever could. Or visit Steve Bates over at The Yellow Doggerel Democrat - he explains it very simpley as well!