Well, wonder no more.
What you are seeing are things called "Banner Exchanges". They can be a good way to get a bit of free advertising.
Usually, a banner exchange works like this: You find a Banner Exchange you like, and you register with it. You send them your banner and "cut 'n past" their little snippet of HTML into your page. Then, you start earning "credits". Normally, you get one credit for every time one of their banners is displayed on your page.
Now, usually, for every two times you display their banner, your banner is displayed on someone else's site. This is what is called the "ratio". A 2:1 ratio is the sort of thing I just described. Some exchanges have 3:1 ratios, some 10:7, some even 1:1.
So, banner exchanges get you free banner advertising. The exchange services let you know how your banner is doing, usually telling you stuff like how many times you banner has been displayed, how many times it has been clicked on, etc, etc.
Also, the really good exchanges also offer targeting. That way your banner that is advertising a football site won't show up on a page about pet kittens, and vice versa. It usualy helps to increase the click-through ratio.
The question is: How effective are they? Well, a common CTR (Click Through Ratio: Means the ratio of how many times your banner is displayed to how many times it's clicked on) is somewhere around 40:1. That means, if the exchange service has an exchange ratio of 2:1, you would have to display their banner 80 times to get one user to your web page.
You might think that a good deal, you might not. Make your own decision.
I have also written an article called "Are Banner Exchanges Worth It?" that I recommend you read.
To choose a banner exchange, head on over and look at some of the banner exchange reviews!