Another artist that I have always liked is the conceptual artist Daniel Dociu, who currently works for NCSoft (He provides artwork for the guild wars games primarily)
This is a good example of his work. Detailed, chaotic scenes with strong palette choices.
This is one of his peices of conceptual artwork for a level that was not used, however the artwork still made it into the game as a loading screen. The basis of the level was a city where the player navigated across the rooftops.
He starts his images with a concept, then starts to create random shapes by using custom brushes within photoshop. He then loks for order in these shapes and bases his images around them.