UPDATED 2/23/07 to fix a crosshair bug.
This is a third person over the shoulder view, a la RE4 or Gears of War.
Unzip the file into garrysmod/lua/autorun, or whatever folder you feel is appropriate.
Bind a key to "chasecam".
It's best for singleplayer games. It tends to have some problems in multiplayer, but it is usable as long as the server has the mod installed.
For more information, see The Official Facepunch Studios Thread.