A very cheap (free) way is to join forums related to the sites theme and post useful replies and thread with a signature linked to your site (if the site permits this).

You could always look at link exchanges, but I feel that these are more useful for getting backlinks and PR to your site as opposed to visitors. Paid contextual advertising such as Adwords (Overture, Cliksor and others) will always get you traffic.

I'm about to go down the press release route with the main site - prweb.com can help you distribute your press release and whilst they do it for free apparently a small donation works wonders!

Some people advocate writing articles and then sending them to article hubs like ezinearticles.com- this way your article (and site link) gets added to various other sites and email newsletters free of charge.

It's also worth making sure that the site has been submitted not only to all major search engines but directories such as DMOZ.org (which powers Google Directory amongst hundreds of others).

Im sure others will offer advice too.

Lyndon