Blender 2.5 now includes a Solidify modifier, used to non-destructively add thickness to thin meshes (similar to 'shell' modifiers in other applications). Solidify allows you to model a simple thin mesh surface, and have a solid thickness automatically extruded by the modifier.
As well as extruding the surface, solidify also has options intended for usage with subdivision surfaces, automatically adding sharp creases to extruded edges and rims.
Possible applications include:
