GlimmerBlocker is no longer maintained and should no longer be used as web sites have migrated from using plain http to using https (i.e. encrypted) which prohibits modification by a proxy unless you resort to install custom SSL certiticates on the client.
You should use a browser extension instead.
It was made as a hobby project during 2007-2008 by Peter Speck. It features both a custom http server and http client and a Mac OS X control panel.
GlimmerBlocker did not only block requests but also allowed for adding custom css and javascript to the pages. It also allowed for modification of the page html before the browser receives the html, which made some modifications much easier than DOM based modifications.
More advanced filtering.
GlimmerBlocker now supports modifying the request before it is sent to the web-server.
A request-modification filter rule can:
Useful links:
Request/response flow. Request modification. Object reference.
Dynamic proxy and dns configuration
You can make GlimmerBlocker use a different proxy depending on the Network location, AirPort network name, IP-addresses, interface state, and more. See the proxies documentation.
Socks5 proxies are now supported in addition to http proxies.
Snow Leopard
Important fix: GlimmerBlocker now supports POSTs and PUTs using content-type=chunked for the request data. This is required for saving files on iDisk (previously uploading failed, or created empty files). Reported by Andreas Maks.
The System Preferences panel can now open without requiring System Preferences to relaunch as the Panel now includes a 64-bit version.
Miscellaneous
You can now copy/paste rules from the 'Filter' tab pane, and drag them into/from other apps.
Rule criterias are extended with path/query 'does not contain', 'not matches regexp' and 'not is'. Suggested by Audun Wilhelmsen.
Better compatibility with ICY clients that expect icy http-headers to be all-lowercase.
Improved checks and reports for broken installations of GlimmerBlocker.
Version 1.4 was released 2009-08-22. List of release notes.