There are a lot of things you can do depending on what skills you have. You can try:
- Affiliate marketing. This is a big one but if you have no capital it might not be a good option. You'll need some money to promote through ppc programs and such, though you don't have to spend money, you can promote through putting up cheap websites, squidoo, article marketing, and other free methods.
- Web design, programming, creative design, copywriting, article writing, any type of freelancing. Depending on what skills you have you can work for other people or companies doing freelance work. Look into sites like scriptlance.com, getafreelancer.com, and odesk.com.
- Publisher programs like Adsense, YPN, Adbrite. You can throw up a blog, forum, or whatever and participate in a publisher program to make money from it. The problem with this will be getting the traffic to your site (especially if you don't have much to spend). I think most people who do this make pennies from it, it takes a lot of marketing and skill to make big bucks with this.
- Sell info products, digital products, etc. You can purchase resell rights to a lot of programs, ebooks, and information in any niche and resell it on your own website. You can even buy PLR for your website at places like easyplr.com so you don't have to do anything. Now you just need to get traffic coming and convert it. On the other hand, you can always make the product and the site content yourself too
Any of these ways can make you anywhere from pennies to thousands of dollars a day. Just please remember never to buy products from sites that promise automated systems, letting you make thousands a month with little to no work, or any type of pyramid scheme or MLM site. If you want to make real money online it takes a lot of hard work and usually a lot of capital too, there's no way around that.