The various web hosting forums are nice to get into, however you'll convert very few clients to your company given that the majority of visitors to those types of forums are web hosting companies themselves.
If you target a niche forum, or a forum that revolves around webmasters in general rather than around web hosting, you'll often find that you will have a higher percentage of signups.
Whenever possible, I give back to the communities that have helped put me in the position I am today. This doesn't mean just advertising the company and a discount for those members, but providing actual information and answering questions users might have. So many new people are starting on the web and forums such as phpbb, vbulletin, joomla, magento, interspire, general HTML and otehr informational boards can be helpful to bring in new business. Again, you're providing information back to the users, and MAYBE you might get a few signups, but this is a very laid back approach to gaining new clients.