I assume that all of you already know about dofollow and nofollow terms. You can click here and check on my previous post about how to detect dofollow blogs. Dofollow blogs lets google spider/bot to crawl on your comments and url to be listed in their search engine.
Another word, the blogs that offers dofollow are sharing their existing google page rank to their commentators. The higher page rank dofollow blogs gives higher credits towards your page rank calculation impact. Basically, commenting on dofollow blogs earns you some page rank juice.
There are a lot ways to find dofollow blogs. Try search “UCommentIFollow”, “dofollow” or “you comment I follow” on google search to get a list of dofollow blogs. Also, make sure you check on the sites whether they really offer dofollow because some of the sites might disable this option.