Here they may pay off. Avoid anybody who promises top ranking listings or overnight results. It takes weeks or months for the registration process and regular monitoring and refreshing to keep good rankings. Some services charge you monthly to re-submit and refresh your listings. I'm not at all convinced this is worth while.
Web advertising is another option.
There are zillions of schemes revolving around banner ads, banner exchanges, sales commissions and other very creative ideas. Web advertising is a specialized area. Anybody can hack up a quick banner and join an exchange service, but specialists can make banners that really get customers attention and target them to just the right audience. For big bucks, of course. Visit Eyescream Interactive for some great banner advice and samples.