A while back I know the owner of this site did a few hundreds $ per month. It was really an eye-opener to me :)
Of course this requires SEO skills to earn more than a few bucks, that need to be learn on the way.
I started a couple of sites like this one, and although I earned a bit of money, I wanted to try sites that are really useful to myself, which I'm focusing on now.
I think it's always good to remind myself that between what would look a half-spammy site to a regular programmer, and the typical too complex projects regular programmers would start, there's a sweet spot in the middle.
Is there a generic webapp (something like OSCommerce) that can automatically create an affiliate store like the one you linked when supplied with a list of item names or URLs?
For most programmers, a project is something that has already a decent size, is complex to get out and test on the field.
If you think this way, a "side-project" can only be rare.
On the other hand: a friend of mine started exactly the way Max describes a long while back, and currently runs way more than 400 sites/projects.
He is also doing way more than 12k€ per month.
The thing is 95% of his projects wouldn't even catch the 'regular' (if there's such thing) programmer's interest :)