I have found a good list of various type of forums which allow DoFollow link. Below is a maintained list of DoFollow forums that allow links in signatures or within forum threads. I have mentioned the Google Page Rank of each forum.

Advertising and Marketing
http://www.experienceadvertising.com/forum/ PR-2
http://www.pakadtrader.com/forum/index.php PR-2

Blogging Related
http://www.bloggapedia.com/forum/ PR-7
http://www.bloggertalk.com/ PR-5

Business, eCommerce, and Marketing
http://www.businesss-forum.com/ PR-0
http://forum.hittail.com/phpbb2/index.php PR-2
http://www.internetmarketingforums.net/ PR-2
http://paymentprocessing.cc/ PR-6
http://siteownersforums.com/index.php PR-3
http://www.startups.co.uk/ PR-5
http://www.smallbusinessforums.org/ PR-5

Computer Related
http://forums.amd.com/forum/ PR-5
http://forums.cnet.com/ PR-7
http://softwarecommunity.intel.com/isn/Community/en-us/Forums/ PR-7
http://www.lex224.com/forums/index.php PR-No PageRank
http://forums.microsoft.com/ PR-7
http://forums.mysql.com/ PR-7
http://forums.oracle.com/forums/ PR-6

Content Management Systems
http://forum.joomla.org/ PR-8
http://forum.mambo-foundation.org/index.php PR-6
http://www.simplemachines.org/community/ PR-6

Domaining
http://www.acorndomains.co.uk/ PR-5
http://www.dnforum.com/ PR-5
http://www.namepros.com PR-5

Gaming
http://legionofsteel.org/ PR-2
http://www.mkpitstop.co.uk/forum/ PR-0
http://forums.worldofwarcraft.com/ PR-5

Loans and Financial Aid
http://www.financialaidforum.com/ PR-4
http://loanofficerforum.com/forum/ PR-0

Media
http://www.movie-list.com/forum PR-4
http://forums.winamp.com/ PR-7

Medical
http://www.studentdoc.com/phpBB2/ PR-5

Motorized Vehicles (cars, boats, motorcycles, etc)
http://forum.autocareforum.net/ PR-0
http://www.autoforumuniverse.com/ PR-3
http://forum.hot4s.com.au/ PR-1

Photography and Video
http://www.photoforum.com/ PR-4
http://opensourcephoto.net/forum/ PR-0
http://www.thephotoforum.com/ PR-4

Politics
http://www.itsallpolitics.com/ PR-3
http://www.politicalcrossfire.com/forum/ PR-2
http://www.politicsforumpoliticalworld.com/ PR-5

SEO/SEM Forums
http://www.davidcastle.org/BB/ PR-3
http://forums.digitalpoint.com/ PR-6
http://www.clicks.ws/forum/ PR-3
http://www.earnersforum.com/forumsindex.php PR-0
http://forums.seo.com/ PR-0
http://www.seo-guy.com/forum/ PR-2
http://forums.seroundtable.com/ PR-3
http://www.sitepointforums.com/ PR-7
http://www.submitexpress.com/bbs/ PR-5
http://www.warriorforum.com/forum/default.asp PR-3
http://forums.webcosmo.com/ PR-0
http://www.webdigity.com/ PR-4
http://www.webtalkforums.com/ PR-4

Sports and Fitness
http://addictsports.com/baseball PR-4
http://www.bodybuildingweb.net/forum/ PR-2
http://www.musclesweb.net/forum/ PR-0
http://www.thesandtrap.com/forum PR-5

Literature (including comics)
http://forums.onlinebookclub.org/ PR-4
http://forums.comicbookresources.com/ PR-5

Music
http://acapella.harmony-central.com/forums/ PR-5

Webmaster
http://www.9mb.com/ PR-5
http://bzimage.org/ PR-4
http://www.dmoz-resources.com/ PR-2
http://www.geekpoint.net/ PR-4
http://forums.ukwebmasterworld.com/ PR-5
http://www.webmaster-forums.net/ PR-5
http://www.v7n.com/forums/ PR-5
http://www.webmaster-talk.com/ PR-3
http://www.webicy.com/ PR-2
http://www.webtalkforums.com/ PR-4

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