Satyajeet Hattangadi has launched a new software called WordPress Commenter. However I do not encourage you to invest in this software as this just helps you to add to the comment spam already harrassing the blog owners and causing a negative feeling about your site.
In any case, most of the blogs have “rel=nofollow” turned on which does not get you any PageRank advantage. Of course, if you make an intelligent comment on someone’s blog by reading the post and adding your own take on to it, it will make you look good in the blog owner’s eyes as well as the other readers’ eyes.
However that requires that you invest a lot of time per blog – not a great idea if you are just looking to get your blog or site indexed.
BTW, if you are sick of comment spam on your blog, activate Akismet. That is a lovely piece of software and can eliminate a large amount of spam automatically. You will need a key that you can get by registering on the WordPress site.