Elevation Data
USGS National Elevation Data (NED) - 10 and 30 meter; SRTM
USGS Gtopo30 - 30 arcsec data - 1000 meter (entire world)
USGS SRTM data (much of the world)
MapMart - free US data downloads, but interface could be better
GeoComm - another free source for US data

Mesh Development Tools
MS FS2002 SDKs (esp. Custom Terrain Textures)
MS FS2004 SDKs (esp. Terrain SDK)
Microdem - geographic data manipulation tool
read_dem.exe - conversion program, DEM/SDTS data to SDK input format

Flight Sim Traveler resources
Excel LOD Calculator (2003-06-22)
Utility to flatten terrain (see Tips for more info)

Forums
AVSIM Mesh Scenery Design - Where I "hang out" on-line
AVSIM Scenery Design - Other aspects of scenery design, some specialized mesh topics
AVSIM General Discussion - For other Flight Simulator information
 
simFlight Forums
FlightSim Network Conferences - FS2004