My no. 50:
50. Age of Empires III (& Expansions) (PC)
Third game in the Age of Empires series of RTS games (fourth if you include Age of Mythology). This one added some new gameplay ideas such as adding mecenaries, fixed trade routes and a levelling home city. Some of these ideas can be seen in the F2P Age of Empires Online.
Clue for no. 49: Started life as a Half-Life mod.
Hmmm... that's probably made it too easy.