I have been using a free software called Blog Commentor Lite.
How it works is that you search relevant keywords using this software, and it will return a list of blog sites that are relevant to your key term. Then you can sort the sites according to PR, and then you simply visit sites and comment.
If you have the time, this is a fast and easy way to create some high PR one-way back links.
I don't remember where I downloaded it from, but try a google search for "Blog Commentor Lite"