WebWasher
Free (for personal use) for Windows, Macintosh and Linux. 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.

KillAd
Freeware for Windows 95/98/ME/NT4. Kills web browser pop-up ad windows.

Ad Filtering programs
Comprehensive list of advert filtering applications.

Zapada
Freeware for Windows 95/98/NT4. Needs Internet Explorer 4.0+, or Netscape 4.04+ (not including Netscape 6). Removes web ads from web pages by blocking any request your browser makes which Zapada thinks is for an ad.

Cut the Crap (CTC)
Freeware for Unix/Linux. Proxy-like open-source daemon written in Python to filter out ads for dialup users.

WebMask
Freeware program for Windows 95/98/NT4. Open source program to remove Web ads. No longer under development by author.

Guidescope
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.

JunkBuster
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.

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

AdWiper
Shareware for Windows 95/NT4. Ad filter for Microsoft Internet Explorer 4.0+ for Windows.

InterQuick
Commercial software for Windows 95/98/NT4. Web Browser filter/accelerator that filters ads and blocks banners.

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

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

nobanner
Freeware for Unix/Linux. Web proxy written in Perl which rewrites HTML on the fly to remove ads.

V6 Web Engine
Freeware for Unix. General purpose Web filtering and processing proxy. Source and binary distributions available.

WebFilter
Freeware for Unix. Web advertising filter based on Cern's httpd web server.

ZeroClick
Freeware for Unix/Linux/Macintosh/Windows 95/98/ME/NT4. Web proxy that closes popup windows and turns banner ads into transparent GIFs. Configurable with regular expressions, pure Java and covered under GPL.

Web Ad Blocking Under Linux/Unix, BeOS, MacOS and Windows
Step-by-step instructions on how to block advertisements.

Adzapper
Freeware for RedHat Linux 6.2/Unix/Windows 95/98/NT4. Open source rule-based filtering proxy written in Python to transform banner ads into transparent GIFs.

Ad Extinguisher
Freeware for Windows 95/98/ME/NT4/2000. Small proxy that blocks banner ads and some web bugs, also freezes animated gifs.

Blocking Unwanted Parasites with a Hosts File
Free for all platforms. A hosts file that blocks most major advertisers, parasites, hijackers and unwanted search engines.

Mike's Ad Blocking Hosts File
Free for all platforms. A hosts file that block major advertisers' hostnames.

Gorilla Design Studio Presents: The Hosts File
Free for all platforms. A hosts file for blocking the hostnames of major advertisers.

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

Rabbit
Freeware for any OS with JAVA. Proxy designed to speed surfing over slow links, by removing adverts and compressing the data stream using gzip and jpegs. Innovative approach.

AdsCleaner
Ad stopper and pop up blocker with additional features. Commercial. Windows NT/2000/XP.

Ad Blocking via CSS
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.

PithHelmet
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.

gozer.org: Mozilla: Ad Blocking
How to use Mozilla and Firefox to create custom CSS rules to hide certain elements based on their attributes and values.