I just started writing here at HubPages, but I have started making a bit of money. I already have some web sites with adsense that were producing some income, but I have seen some incremental add revenue since I started posting hubs. I have also made a little bit from the HubPages adds as well. Nothing to write home about, but it a few bucks, and I assume that, like my web pages, they will generate more traffic as they are indexed by the search engines.
My main driver was to build a bit of residual income, mainly to diversify my income. I don't have any expectation that I will replace my full income here, but income streams away from work provide for flexibility and I do enjoy writing, so it has dual benefits.