We take more photos than any family I know, enough that our kids probably think their parents have lenses attached to our faces. When you take that many pictures, some of them are bound to turn out to be interesting enough that somebody else might want to buy them.
I have no interest in going through the trouble to get extra change from this, but a friend at work recently wrote up his experiences selling to sites like Shutterstock and iStockPhoto. He makes about $150/month, on a few hundred images he's uploaded over the years. Basically these sites give you a cut (25 cents or so) per photo their users download; users are charged from $1 to $10+ depending on the resolution.
Most of the pictures people buy, apparently, are travel-related; something to consider if you go on trips to places like Switzerland, Argentina, or China.