Geocities now uses these instead of pop-up ads. Basically they are a little square of advertising that stays in the upper-right of your browser window. These forms of advertising can be easily closed with two clicks in the same place, one to minimise the window a-la Winamp, which also changes the button to the familiar "X" close one. Unfortunately the square will re-appear once you go to a different window, so I'm not sure whether it's worthwhile to close them or not.

I'm still not sure whether I prefer pop-ups or the new and "improved" ad-squares.