GlimmerBlocker is no longer maintained and should no longer be used as web sites have migrated from using plain http to using https (i.e. encrypted) which prohibits modification by a proxy unless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.
Compability with Entourage/Exchange? (Microsofts NTML authentication scheme "extends and embraces" the http protocol). Symptom was that it didn't accept the credentials when logging in. Problem reported by Steve Lombardi.
Compability with getdropbox.com which uses the 'connect' http method to port 80 instead of plain http when using an http proxy. Reported by Steve Lombardi.
Better compability with Little Snitch. It is now able to recognise the proxy server as "GlimmerBlocker proxy" instead of just an unnamed java process. Requested by Jean-Claude DE SOZA and William E. Delcomyn.
Compatible with HTTP/0.9 servers, an obsolete version of HTTP. The "D-Link ADSL Router" uses HTTP/0.9 when responding to the login request even though it uses HTTP/1.0 for all other responses. Reported by Jean-Claude DE SOZA.
Fixed handling of "Expect: 100-continue" http transactions. Used mostly by Microsoft based software such as iSpQ video chat and some IIS servers (seen in video streaming from dr.dk). Reported by Daniel.
Improved handling of QuickTime RTSP over HTTP. Previous versions closed the control POST connection too quickly. RTSP POST handling is now enabled for all user-agents and not just the QuickTime Player. Reported by B. Kuestner.
Fixed problem updating Network Proxy Settings when an interface did not have a proxy protocol attached (interfaces which cannot use proxies). Reported by Guido Berger.
More compatible with servers that sends invalid response headers. Some servers repeat the http status line in the middle of the http header response (e.g. 'PePLink Balance'). Reported by Daniel.
Fix bug in Network Proxy Settings Updater which would crash if a network interface did not have a proxy protocol attached. Reported by Guido Berger.
Fixed "duplicate filter-id" bug which happened when adding another filter subscription before quitting and relaunching the Preferences Panel. Reported by Jean-Claude DE SOZA.
GlimmerBlocker can fail if an obsolete java xml parser is installed in /Library/Java/Extensions?. If configuring the xml parser fails partially it now prints a message in the log about having obsolete java libraries installed.
Version 1.3 was released 2009-01-20. List of release notes.