Place adverts on your site to earn some money. You can use third party services like Google ads- which are easier, but you have very limited control, and, of course, the provider takes their percentage. If you don't like sharing your hard earned revenues, then try one of these:
OpenClassifieds is a free open source script for classifieds, advertisement or listings. This web application is developed to be fast, light, secure and SEO friendly. Template enabled and easy to administrate.
Prosper202 provides pay per click affiliate marketers with leading edge self hosted ppc software. Prosper202 is committed to making the latest pay per click marketing technology available to our users (such as real time tracking). You can rest assured that with Prosper202 you will be on the cutting edge of the technology available.
Noah's Classifieds Free Version is a great Classifieds App that can be used for any niche.
OSClass is all you need to easily create your own classifieds website. It's a free and open script to create your advertisement or listings site.
Revive Adserver is the open source ad server formerly known as OpenX Source. Revive Adserver enables publishers, ad networks and advertisers to Serve ads on websites, in apps, and in video players, and collect detailed statistics about impressions, clicks and conversions.
GPixPixel is a FREE yet powerful million pixel script, based on the popular marketing concept. Advanced settings include, among others, the ability to use interlaced images, temporarily take the site down for maintenance and put it back online when all changes are done, and enable/disable blog comments. Multiple payment modules available for pixel purchasing, including Authorize.Net, PayPal, NOCHEX, PSiGates, and others. More are being added all the time. PayPal IPN is fully supported, too!