Here is a list of dofollow forum which have Google page rank, register there use your blog link as signature and make sure that you stay active as these forums are very active and you will get backlinks as well as traffic.
I suggest you to keep checking this forum for Dofollow attribute because sometime webmasters remove the dofollow attribute because of spamming. and also its not possible to be active everywhere but I will suggest give a visit to every forum and select 3-4 forums of your choice according to your niche and stay active. Add values to the people post by commenting.
5. http://forums.searchenginewatch.com/
6. http://forums.cnet.com/
7. http://forums.mysql.com/
8. http://forums.digitalpoint.com/
9. http://affiliate-marketing-forums.5staraffiliateprograms.com/
10. http://www.bloggertalk.com/forum.php
11. http://www.sitepoint.com/forums/
12. http://warriorforum.com/
13. http://filesharingtalk.com/
14. http://freeadvertisingforum.com/index.php
15. http://www.geekvillage.com/forums/index.php
16. http://www.submitexpress.com/bbs/
17. http://www.htmlforums.com/index.php?
18. http://www.submitexpress.com/bbs/
19. http://www.siteownersforums.com/index.php
20. http://www.directoryjunction.com/forums/