Plug-boards are places where you can add a banner or button for free. As you add your button the last button on the board moves off, so it's not permanant advertising, but you can get lots of curiousity hits from posting on these. The most common banner/button size for these is 88x31--though there are plug-boards taking other size banners, such as 468x50. Plug-board sites usually have rules against posting adult sites or posting multiple plugs plugs (though they usually allow you to plug again once plug is "knocked off" the board). All the plug-boards on the list below allow commercial sites (at least last time I checked).