Internet Junkbuster is a free (this does not refer to its cost, free as in freedom), GPLed privacy proxy. It has been created by the Junkbusters, an american privacy group. It features, among others, regexp-based ad filtering and selective cookie blocking.
The original version can be obtained from Junkbusters. Here is my blockfile, as of 18 July 2001.
When your browser asks for an URL that you decided should be filtered, it sends an HTTP "Temporarily unavailable" error message. That was fine for me, until I discovered that Netscape 4.x, in certain circumstances (met on, e.g., hardware.fr) put the error message on top of the text of the page one is trying to read. So I decided IJB should send a tiny image that would take the ad's place more neatly in all circumstances. So, I drawed an ugly tiny GIF and made an horribly ugly hack to the code so that it sends this GIF instead of the error msg.
Here you can have my modified version, but you'd better have a look at another fork, privoxy; it is much better, and is what I am using now. If you still want my modified IJB, here you are: