Xplanet was inspired by Xearth, which renders an image of the earth into the X root window. All of the major planets and most satellites can be drawn, similar to JPL's Solar System Simulator. A number of different map projections are also supported, including azimuthal, Mercator, Mollweide, orthographic, and rectangular.
Didn't mean to grumble ...
Lest I be misunderstood, FUDforum is a great piece of software. The support forums are very good and the developers are extremely responsive.
Integrates with mailing list
FUDforum can be used in conjunction with a mailing list, so subscribers can get every message posted to the forum by email as well as post new messages by email.
None of the other forum software I looked at had this capability, which was important to me.
Unfortunately forum members must make two accounts, one for the forum and one for the mailing list since they are two separate entities.