A simple skeleton BHO that from Pete Warden:
http://petewarden.typepad.com/searchbrowser/2007/05/porting_firefox_1.html
A more complex example based on PeteSearch:
http://petewarden.typepad.com/searchbrowser/2007/06/porting-firefox.html
From Sven Groot, the source to his FindAsYouType extension:
http://www.ookii.org/software/findasyoutype/ http://www.ookii.org/download.ashx?id=FindAsYouTypeSrc
The SurfHelper pop-up blocker from Xiaolin Zhang:
http://www.codeproject.com/shell/surfhelper.asp
A simple example covering all the BHO events from René Nyffenegger: