Hyena is a C# library used to make awesome applications. It contains a lot of random things, including useful data structures, a Sqlite-based db layer, cool widgets, a JSON library, a smart job/task scheduler, a user-query/search parser, and much more. It's particularly useful for Gtk# applications, though only the Hyena.Gui assembly requires Gtk#.
It is used by Banshee, PDF Mod, and F-Spot.
The best place to discuss Hyena for the moment is the Banshee mailing list or IRC channel.