Whether you are interested in developing plug-ins, using plug-ins, or just interested in better ways to use Finale, you should check out Jari Williamsson's Finale Productivity Tips.

Perhaps the most extensive single Finale Plugin product is TGTools by Tobias Giesen. Some of my favorite functions in TGTools are

And many, many more.

Recordare is devoted to MusicXML. They offer a Finale plug-in that converts Finale files to and from MusicXML. One interesting thing about it is that this plug-in provides by far the best currently-available solution for porting a Finale file to a lower Finale version. It also provides the most accurate means of converting between Sibelius and Finale.

Matthew Hindson has a number of interesting fonts on his site.