A new'ish site allows you to make graphic (or text) ads and place them on blogs and websites all over the place. It's been around long enough to be established, and the crafters worls is very involved in it.
What: You register for free, make up an ad, there are 6 or 7 sizes from the 'button' to 'skyscraper' ranges. Use a great photo, catchy phrase or whatever.
Why: The major objective is to allow others to receive income from ads on their site, from anywhere like 1 penny to a dollar or so a day. (Really busy sites can charge the higher rates, but even little guys can charge the lower rates). But the additional reason is for those of us with webshops to place ads. You specify the cost you are willing to pay (for me that is $0 right now). You then place a 'bid' and await approval from the site owner. There are many sites with free slots that no one else is bidding against you.
Who: Anyone can make an ad and bid on a site. If you are able to work this blog, you can make an ad. It's very easy. And the site contains a search function to find folks who will place your ad (for free, or you specify the price you are willing).
When: The free ads are good for 2 days. OK, it's not long but I am already on two sites and loving it! (An Etsy blog, and a vegetarian site).
Don't hesitate to message me if you need help.