I would guess that the core of the game development community is well represented on the two big sites,
GameDev.net and
Gamasutra. I think this here JavaGaming.org is the definitive Java-specific game development site.
Naturally, different technologies have their own homes, and small communities tend to grow all over the place.
Dexterity Software has a good set of interesting articles and a lively forum. I wouldn't say that
OpenGL.org has a community as such, but it has a useful set of
forums.
As for potential customers, the
Association of Shareware Professionals should be a good place for users to find software, but it's woefully under-advertised. The afore-mentioned Dexterity is a popular site, as are
Download.com,
Popcap and
Real One Arcade.
And many, many others...