WebMask is a program for Windows 95/98/NT which removes garish adverts from web pages. It also provides extra privacy while browsing, by hiding information that the browser otherwise reveals about your system.
WebMask is open source - it is free for you to use, except for certain conditions which exist to ensure the freedom of the software. See the GPL for full details.
WebMask is still in beta testing. You can download it and try it out. To find out more before you download, you can read the help pages online.
If you don't run Windows 95/98/NT then look at the links page to find out about other similar programs.