I make some pocket change with Amazon.com's affiliate program. I have a few websites about my interests and I always link to specific books and get a kickback if anyone clicks through an ends up buying them (or anything on Amazon after clicking through my links, for that matter). Back in the day, I used to be more aggressive--writing book reviews, visiting forums, etc. but now I don't give my sites any PR and I still get about $25 every quarter.
I think I could do more if I wanted to but I already spend 8 hours a day at the computer; I try not to assign any more of my time to stationary pursuits.
If I find that I'm having trouble paying myself first with the amount I've designated, I think I'll take up being a waitress or clerk on one of my days off to make a little extra and interact with people more.