If you sign up at
Spottt, you get your advert shown on other sites for free. It's really easy to set up. All you have to do is create an ad 125x125px and upload it, then add their code to your page.
The more the page you post the code on is loaded, the more times your ad will be shown on another site. That's great if your site gets 100's of hits a day, but for site's with few members (like mine) it could take a while, however I've found a little cheat. After you have been approved (took about an hour for me).
Go to
Refresh This and paste your URL in the box, set the refresh rate (I use 10 sec) then just let your browser do the work. Remember not to close the tab that refresh this is running on.
Although the numbers aren't huge it's a start. Today my ad's been shown 66 times, and since I started I have four new members, and a couple of clicks on my Adbrite ads. Which is more than I was getting before.
Hope it helps.