If the site is really good, it should spread from person to person by word of mouth. ie: Napster.
You might want to start with search engine submit, with good choice of keywords. There are many sites devoted to this , just search for "Search engine optimization."
You might try to find relevant sites that you can submit your link to.
And of course, add your site to your forum & email signatures, any profiles, and whereever else you may go on the 'net.
If you have the money, you can buy banner space on sites.. of course, you would really have to know what you're doing.