Here are a Few ideas, none revolutionary but may offer some inspiration.
Informative website – Build a site using your own knowledge on certain subjects. Depending on the popularity of the chosen subject, the level of competition and the quality of your content, this can return a steady monthly income from affiliate marketing and advertising.
Reseller site – Some sites (most notably Amazon) offer potential for third party resellers to make money from selling their products. This can generate an small income if the site has enough traffic.
Directory Service – There are many web based directories that have become successful. Due to the amount of competitors you are unlikely to make a fortune from charging people to list, but money can be made for charging companies for higher priority listings.
Facebook Applications – Facebook has an API available for developers to build applications. Some applications have millions of users, showing adverts on these applications can generate a substantial income.