Signatures at webmaster and other forums were always used for creating awareness about your site. Avatar advertising is another effective way of brand promotion.
Just check out Digital Point. This is a hugely popular webmaster forum, incase you weren't aware. Just check out the profiles of some members with greater than 500 posts. Most of them will either sport a site-sift or sevenseek avatar.Some of the generous souls did it for free, while the others were lured with $$. Cool way of advertising, IMO.