are you looking this for your website promotion, to get some more back links? It is not good to use blogs and forums for your very own benefit, just for the sake of some back links. If you feel like you have something to share with the blog owner or to the members of the forums then you should use these mediums.
By the way there is no short cut to promote your website so I'll suggest you to not look for a dofollow blog or forum instead write some good articles and PR to submit to Article and PR directories, do blogging yourself etc. to promote your website.