Hi,

this is a list of few forums which allow dofollow signature from the first post.

CashFindForum - HYIP, Forex and other online investments related discussion board.
Making iMoney.com - HYIP Forum for Making Money Online
Webmaster Forum
Technology, Web & Business Forum - Powered by vBulletin
Make Money Online Forum - EarningeMoney.com
Article Marketing SEO Forum :: Index
Graphic Design Forum - A Platform for Graphic Designers
Seobetter.com - Index page
DreamTeamMoney Internet Marketing Forum | Webmaster, HYIP, Forex, Network Marketing, E-Currency, Money Making Forum
Site Owners Forums - Webmaster Forums
Internet Marketing Forums
Helping Webmasters - Webmaster Forum, Website Support and T35 Hosting Support - Index
forum-3.com

I hope it is helpful to you.