Ground Environment (replacement textures, new autogen, and more)
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=ground_environment
Ultimate Terrain (three parts, new landclasses for Europe, Alaska/Canada, USA)
for example:
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=ut_europe
Flight Environment (replacement textures for clouds and skies, sun textures, new weather effects - absolutely fantastic and sometimes most beautiful)
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=flightenvironment
And finally diverse local sceneries and airports sceneries for the regions you fly in most of the time, for example
http://www.aerosoft.com/cgi-local/us/iboshop.cgi?show70,4609965590
Monacco:
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=monaco
Helgoland:
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=helgoland
German Airports series:
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=ga1fs2004
Frankfurt:
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=megaairportfrankfurt
Satellite picture sceneries:
http://www.aerosoft.de/_php_projekte/_php_screenshots2/screenshots.php?sp=fs2004&p=realgermany3
There is so much more. Wile I refer to german scenery for the most (no wonder, I live here), pendants are available for Europe and America as well. Air picture sceneries have no season- and time of day-dependant weather effects anbd lights, since they are pohotos you are flying over, keep that in mind. such sceneries are available for parts of the US, complete Netherlands, Britain (for example:
http://www.aerosoft.com/cgi-local/re/iboshop.cgi?showd20!0,4609983870,10306 ), major parts of Germany, complete Switzerland. there is also a FREE Netherlöands scenery, check the news at AVSIM.com.
Beside German Aerosoft, this is also a major player in commercial products:
http://www.flight1.com/products.asp?category=scenery
I bought FS9 plus Ultimate Terrain 1-3, Flight Environment, FS Global 2006, Ground Environment and some aircraft when FSX was released, and I bought it all via Ebay, saving me 50-60% of the money I would have lost when buying new in stores. Will upgrade my German Airports 1-4 and Austrian Airports next.