I quitted my job 1 year ago and basically set out to make my own living on the Internet. I went from making an adsense-supported website offering free gift certificate templates to a Body Mass Index calculator with affiliate revenue, an online guide to help choose travel insurance, and now a SAAS offering for sending large files for small companies.

All of these websites make money to some extent. However I found the process of developing niche websites not to be too enjoyable (lots of focus on specialized content, nitty-gritty of building links for SEO etc) which is why I now try to focus on the SaaS file sharing solution.

If you want some money on the side, then niche sites are a good way to educate yourself on how to do it. On the matter of how to choose a specific niche, I found the ebook by Rob Walling (start small and stay small) to be a real eye-opener : http://www.startupbook.net/

Also, I would recommend you to check out http://www.flippa.com every week or so. You can see which websites are selling, and try to understand why they're successful and hopefully using this knowledge to advance your own ideas.