This was a side-project of mine that sort of happened by accident... but helped pay my way through Berkeley! It started out with me goofing around one night with Wordpress. I wanted to see how easy it was to create a blog on there versus a few of the other blog platforms, and before I knew it I had set-up an affiliate network and was feeding products to the site that paid commission on sales.
It was a great experience in learning first-hand how to set-up product feeds, optimising for traffic, and creating valuable content that actually lured people in for interest, but ended with them making purchases they were stoked on. In the end, working full-time and studying part-time trumped giving the site the time it deserved and required, so I shut it down.
The most valuable take-away from this site was how easy it is to build something from scratch, and how much fun it is to work in a virtual world with no office, warehouse and limited overheads.