As everyone suggested google adsense is best, but getting approval is very difficult. Before that you can go for other options such as infolinks, adfly, facebook, twitter etc, also try to promote legal stuff.
i use a free ppc ad exchange network. openadzone.com offer me quality targeted traffic (GEO, KEYWORD). Both text and flash ads available.
Dont be dumb and use adsense for illegal content. They will let you earn for a couple months then they will close your account with a automated email and keep all your cash. but you can sue them in small claims court and get all your money back but why go through this