For several months I have been trying out two different small time ad networks to see if they were viable methods to produce extra income. The first one (Adbrite) went out of business after about ($10 of income over about a year). The second one (Project Wonderful) I really did like the idea and service, but after about a year of having their ads run on a few of my sites, my total income from them is around $8. So in the end It looks like they were not even worth the time I spent installing their ads.
Also, just for archival purposes, (as I am not actively tracking the changes in my overall income per hour) currently I estimate I have earned about $0.91 per hour I have invested in this experiment.