My list includes some redundancy (2 forum systems), but find that all in all, I can get most sites done without 3rd Party add-ons.
Mambot:
RD_Readmore from Run Digital (
http://www.run-digital.com/download/cat_view_4.html )
Why: Allows greater flexibility when using the Read More. Particularly if you modify the file slightly, you can then create red more links in the format "your text" + title alias + "your text". This increases accessibility.
Component:
Simpleboard/Joomlaboard
Why: Good basic forum component if you want an easily integrated forum. Solid, plenty of good features.
SMF Bridge
Why: If you need a more solid forum and want it integrated. Requires a full stand alone install of SMF.
Bookmarks
Why: Provides more flexibility than the build in weblinks component
Modules
Extended menu (
http://de.siteof.de/extended-menu.html )
Why: Allows enormous flexibility in menus, including the use of flat lists that display children items (submenus), Access Keys, link titles, which is not currently available out of the box. Increases accessibility tremendously.
I can't think of any other ones I use on a regular basis...