Freeware for Unix. Caching proxy which can block ads. You can also use squidGuard and SleezeBall to help do this.

Freeware and commercial versions for Windows 95/98/ME/NT4/2000. Runs in browsers java virtual machine and kills ads and cookies, slowish.

Shareware for Windows 95/98/ME/NT4/2000/XP. Small, patch for common web browsers. Removes banners, buttons, popups.

Freeware for Unix/Linux/FreeBSD/OS2/BeOS/Windows 95/98/ME/NT4. Web proxy that filters banner ads, cookies, and referer tracking. Supports wafers and User-Agent spoofing.

Free for non-commercial use, for Linux/Sparc/Solaris/Windows95/98/ME/NT/2000. Proxy that filters ads by pattern matching and blacklist. Also filters cookies and referers.

Free for all platforms. A hosts file that blocks most major advertisers, parasites, hijackers and unwanted search engines.

An extended site preferences and ad blocking plugin for Apple's Safari browser. The basic purpose of the plugin is to allow the user to control the blocking of ad images, Flash, Shockwave or midi loops.

Donationware for Windows. Ad filtering proxy which Includes Cookie, Popup, Animated Image, Referrer and Webbugs filter. Has automatic browser configuration for Netscape 3-6, Internet Explorer 3-5 and Opera 3-5.

How to use CSS rules to block ads. The technique works for any Gecko-based browser and for Apple's browser, Safari.  It even works with the Thunderbird mail client.

Commercial software for Windows 95/98/ME/NT/2000. Ad remover, automatically configures browser, and comes in five European languages.