The worst bunch of replies ever. All but 1 reply was off topic.
The best place for FREE advertisement at the momment would be FaceBook.
Create a fan page. A few posts a week.
Find pages in your niche(web games, mafia games, pirate games, or we). Contact Admin of the fan page in hopes a free post.
I even created a seperated FaceBook account and been slowly building friends everyday that are gamers. Involves a lil more work then just friending anyone. Post random fake **** about my life. About once a week, ill post a link to one of my sites saying i found a cool site. :)
I also use twitter, i get traffic and retweets, no noticable conversions.
For paid, I would seek owners of similiar unrelated sites. for ex: if your game is about pirates, seek pirate fan sites and blogs, forums. Also find related pirate cartoon and anime fan sites, blogs and forums. Contact owner about paying him/her for a banner ad in header for a flat fee per week or month.
Also find similiar facebook fanpages and if Admin will not do it for free, offer to pay. :)