XAKT Media has added Expression Web support for many components of its InstantFX SE software. Add Flash navigation buttons, Flash MP3 players, slide shows, and more to your Expression Web pages. You can find its site here:
http://www.instantfx.net/
Themed Web site templates help you build and launch a Web site faster. The templates are designed for Expression Web and provide themed, prebuilt sites complete with dummy pages and valid style sheets:
http://www.i3dthemes.com
More extensions and add-ins for Expression Web will likely be popping up. Type Microsoft Expression Web extensions in your search engine to hunt for new goodies.