It is found a total of five times in the level Temple Ruins where it is used to unlock gates.
Locations of the Keys
The Keys Of Ganesha are scattered around the level, and the first two of them are used to open a gate leading further into the level. The last three are used to open the level exit.
The second Key Of Ganesha is obtained after completing a puzzle involving several movable blocks. This floods one pool with water, and then Lara is able to pull an underwater lever, opening a gate to where the key is.
The fifth and the last Key Of Ganesha is located in a pool of water, accompanied with a deadly current that will pull Lara to the spikes. The goal is to set off the current by pulling two levers, which will enable picking up the key.
Ganesha or Ganesh is one of the best known Hindu deities, easily identified by his elephant head. 
- http://en.wikipedia.org/wiki/Ganesha Retrieved on 24th Sep 2009