This tool allows the user to build, and edit maps from games that are a
Half-Life 2 game or modification. The program is pretty much like the
Developer's Kit, that all Game developer's use, but is made to be a little easy
for the user to use. But Source SDK is also sometimes hard to use. Not
only can you edit maps with the program Hammer, but you can also use
the Model Viewer to edit models from Half-Life games. But most of all
Source SDK is a free program that will allow you to construct, and edit
about any map that's HL2 formatted.