What are the essential add-ons? What are the add-ons Celestia users should use (almost) regardless of their system specs?
My current short list:
The Extended Stars Database : http://perso.wanadoo.fr/celestia.stars/
Selden's Catalogs : http://www.lns.cornell.edu/~seb/celestia/catalogs.html
I'll add more as I remember where they came from. But I think this is a good start.