Texture Atlas
When many separate textures are packed together into a single texture, this is called a texture atlas. Other common names for this are decal sheet, packed texture.
CategoryEnvironmentModularity has examples of texture atlases, and atlas planning methods.
Work-flow for large texture atlases from the Polycount forum
UV tiling tricks within 0 to 1 space from the Polycount forum
Practical Texture Atlases on Gamasutra by Ivan-Assen Ivanov
See also TextureCoordinates and LightMap.
Gutters
When a game model is textured using a single texture sheet, the texture will have typically have UV'd areas (UV shells) and blank areas between them (gutters).
The width of the gutter needs to be wide enough to accommodate how much EdgePadding you need. This varies depending on how large the texture is and the texture filtering type used by the game renderer.
Packing Tools
Decent UV Packing Script/Tool? thread on the Polycount forum.
Headus UVLayout ($)
UV Master script for ZBrush
TexTools script for 3ds Max
Xrayunwrap ($) script for 3ds Max
Texture Atlas Generator script for 3ds Max (creates an atlas by slicing up the mesh)
Normalize UVs Maxscript for 3ds Max
Texture Atlas Tools by Nvidia