How about telling Google to recommend us some useful sites for this purpose? http://www.google.com/search?q=dofo...avclient-ff&ie=UTF-8&rlz=1B2GGGL_enSG202SG202
More useful is this.... http://images.google.com/images?hl=en&q=U+comment+I+follow&btnG=Search+Images&gbv=2 what do you say?
If you're trying to leave links by commenting on blogs or dropping links in signatures, then trust me when I say it won't help you much SEO wise. If you become a respected member of the community however, they can do wonders for your traffic. Just thought you'd want to know.
Wait, dofollow blogs are useless?! Ok, ok, I get atleast one sarcasmic post a day! I would really avoid all the "simple" methods to build irrelevant, non-quality backlinks since they are a waste of time. Most search engines can detect "blog comment spam" and de-value the links most likely if not for sure. Work on quality relevant backlinks, remember 50 of these are better than 5000 of the other.