Well, there are also tons of Free For All Link Lists, Lesser Search Engines, General Directories, Product Directories, Service Directories, etc...

And of course there are all of the companies that will charge you to submit your site to all of these (and some charge quite a bit, depending on number of site submitted to and frequency of re-submissions). To be honest, results that you get from this can vary greatly and are usually best for local or smaller businesses, moderate for forums, and lesser for online bussiness like hosting providers.

Although our company has a service like this as well, its not something that we mention on our site due to the possibility of poor results/moderate cost.

To be honest, I wouldn't really recomned submission services as a good place to start either...

However, what may be worth your time is looking for specific "Free/Open" forums/link/directory sites that specificaly relate to your Forum's topic...

Also, you might want to think about submitting your URL to things like dmoz.org ( http://www.dmoz.org/add.html ).

I hope thats sort of helpful....