Wow - there are a million things like that - welcome to the soft, seedy white underbelly of the internet!
There is a whole world of what is called affiliate marketing. Do a Google search for it and you'll find that there are companies that will pay you for obtaining a zip code, an email address, getting someone to sign up for a trial offer of something, etc. Big companies are Commission Junction, NeverBlue, and ClickBooth. There are literally hundreds of these.
Now, you need a place to get traffic / potential users / customers. This can range from PPC on Google / Yahoo / Bing, 'review' sites (for example, Netflix compared to Blockbuster, with an opportunity for the person reading the reviews to sign up for a trial of either offer) to YouTube videos with your web site on them, to media buys / banner ads, driving traffic through Craigslist (frowned upon, but done by some) - the list goes on and on.
Time for dinner - if there is interest on this thread I'll respond in more detail later. -e-
EDIT: Note that there are some ways to autocreate sites that do $1 a day (autoblogs with Adsense / banners, etc), but the bar should be higher for anything meaningful. My threshold was $50 / day / site when I was doing a lot of this. I've abandoned this approach to build a 'real' startup, but it is pretty easy for someone to build up a $100/day income leveraging traffic in a relatively targetted way from web 2.0 sites.