You know that link building is one of the most importent thing for SEM. You can get some backlinks with posting comment to other blogs. But some blogs are nofollow.
What is nofollow?
Some people are spamming blog comments with their comments and to avoid this there is atribute NOFOLLOW in HTML. If you want to protect your site from spam you can make links in comment nofollow.
<a href="http://yoursitename.com/" rel="nofollow">Anchor text</a>
rel='nofollow' - when search engine see this atribute it don't count it as backlink. But there is blogs which are Dofollow
What is Dofollow?
Dofollow links are links which are count as backlinks by search engine. When there is no atribute rel or there is atribute rel='dofollow' it means that link is dofollow and it is counting by search engine.

Here are list of dofollow blogs:

Can make big money online - http://www.canimakebigmoneyonline.com/
Compton SEO – http://www.comptonsolutions.com/
Communityspark - http://www.communityspark.com/
Bull3t’s blog - http://blog.bull3t.me.uk/
Computer Chit Chat - http://www.computerchitchat.com/
Communityspark – http://communityspark.com/
Daily SEO – http://www.dailyseo.org/
Chexed - http://chexed.com/
Writing Help - http://cutewriting.blogspot.com/
A List SEO - http://www.alistseo.com/
SEOptimise - http://blog.seoptimise.com/
Webicy Blog - http://blog.webicy.com/
Link Building Bible - http://linkbuildingbible.com/
Tips and Tricks Fom Moms and Dads - http://worldofsolution.blogspot.com/
Acakadut - http://acakadut.com/
Getblogged.info - http://getblogged.info/
Daily SEO – http://www.dailyseo.org/
Imran Khalid - http://www.imrankhalid.com/
Child Development - http://community.child-central.com/
Local SEO Guide - http://www.localseoguide.com/
Woman Start Your Business Now - http://womanstartyourbusinessnow.com/
Home Based Business - http://www.bestrealincome.com/
Review Blog - http://reviewabc.com/

if you have dofollow blog you can comment it and i will add it in list. My blog is Dofollow too.