GlimmerBlocker a different type of ad-blocker. Other ad blocking tools for Safari are often implemented as horrible hacks, such as an InputManager or ApplicationEnhancer. This can cause problems for Apple when they release new versions of Safari. It also compromises the stability and performance of Safari. GlimmerBlocker works as an HTTP proxy and Safari's stability is not affected. It doesn't use hacks. It works with all browsers.
You can upgrade Safari without breaking GlimmerBlocker or waiting for a new release. GlimmerBlocker can also be upgraded without having to upgrade Safari. This makes it easier to use Safari's beta versions and WebKit's nightly builds. GlimmerBlocker isn't capable of hacking Safari. It can add a block by right-clicking on an image, stop pop-unders and filter cookies from third-party sites. You do gain stability. GlimmerBlocker makes it much easier to modify pages yourself by adding CSS rules, JavaScript, or transforming the HTML before Safari gets it. I hope you are happy with the tradeoff.
JavaScript is a programming language that allows you to modify pages. For examples, see the filters, e.g. adding a YouTube download link.
Version 1.6.6: