It's under active development, including adding features to stay mostly compatible with Second Life. Bakes on mesh will probably be the next big feature. Firestorm is probably the most popular viewer. I think BulletSim is now the default physics engine. There are two grid-independent currency systems, Podex and Gloebits.

I heard somewhere that there are around 300 public grids, most of which are on the hypergrid. There are a few significant forks, most notably Halcyon (created by the late InWorldz LLC) but there are only a handful of grids running those.