Some old, some new. Most are devoted primarily to already-written extensions for downloading. All those included here have at least some how-to information as well.
http://www.macromedia.com/support/dreamweaver/extend.html and http://www.macromedia.com/support/ultradev/behaviors/create_extensions/ The official Macromedia help sites for developing extensions.
http://www.udzone.com The UltraDev zone. Includes brief tutorials on writing extensions, along with some helpful tips.
http://www.basic-ultradev.com/building_extensions/index.asp Online companion site to Building Dreamweaver 4 and Dreamweaver UltraDev 4 Extensions, by Tom Muck and Ray West.
http://www.andrewwooldridge.com/dreamweaver/ Andrew Wooldridge's Dreamweaver Depot. Andrew and his site have been staples of the extensibility community for quite awhile. Check the Tutorials link for a few project-specific tutorials.
http://www.massimocorner.com/ Massimo Foto's Dreamweaver site. Massimo was voted Best Extension Developer at the Macromedia Dreamweaver Conference 2000 in Monterey, California. Includes a Tips & Tricks section.
http://home.att.net/~JCB.BEI/Dreamweaver/ The Excellent Dreamweaver Supply Bin. Mostly devoted to offering third-party extensions, this site includes a very basic overview of how "inserting stuff in Dreamweaver" works.
http://www.arrakis.es/~andrewc/downloads/dream.htm Andrew Castle's Dreamweaver site. An oldie, but a goodie. Includes a few tutorials on specific projects and an overview of how objects and behaviors work.
http://www.extending-dreamweaver.com/ This site is devoted to extension developers. Resources include shared scripts, shared C libraries, user interface and general extending guidelines, and some utilities.
http://www.geocities.com/rabi.s.raj/dreamweaver/ Rabi Sunder Raj's Dreamweaver Extensions site. Rabi's Menu Builder won the Best Extension award at the Macromedia Dreamweaver Conference 2000. No tutorials, but a How-To link to the WebMonkey article on extension development.