I tried out something called Project Wonderful the other week, PW is an advertising network, you can buy and sell advertising space across a network of sites and blogs, what you try to do is outbid the advertiser that is already filling an ad space you desire, if you outbid them you'll take over that ad space for as long as you set it for or until someone else comes along and outbids you. You only pay when you win a bid and only for how long your ad is served.
I tried it as an advertiser.
The directory of sites you can choose isn't really that varied, the majority of sites seem to be for online comic strips, but I found it great for a few of my blogs topics. What I found interesting is that literally most of the sites have very low bids, maybe 10-20 cents each and with the amount of sites you could possibly buy a lot of cheap advertising pretty fast.
I tried out a couple cheap bids along with a higher bid, I did get traffic from all of them, though not a huge amount, but what I'm thinking is that if you buy a lot of ad spaces the traffic will ad up.
The statistics for the publishers sites was also pretty cool, they have daily and weekly estimates of traffic for each site in the directory along with graphs showing bid performance and previous traffic reports over previous months, it gives you a sense in knowing how an ad of yours could perform.
I haven't used PW to try and sell advertising space on any of my blogs yet, but I'm thinking about starting soon. Even with small bids on your ad space, if you can fill em you could regularly make a couple bucks a day and even more if your in demand.
What led me to PW was that I was looking for a cheap easy alternative to adwords, and it's certainly cheap and easy, still got a few bucks in my account and I can withdraw that back into my paypal account anytime I want as long as it's over ten bucks, I can even use money earned from selling ad space into buying more ad space myself.
It's a very flexible easy and cheap service to use and actually pretty fun, I never knew how big the online comic strip niche was.