In celebration of Overkill reaching 1.5 Million members in the Payday 2 group on Steam, they will be giving away free copies of the original Payday for 24 hours, beginning Thursday, October 16th @ 10am Pacific (1pm Eastern). Ordinarily, the game costs $15.
Reviews were generally favorable for Payday when it first came out, and it currently holds a 76 score average on Metacritic. Since it's going to be free, there's really no excuse to not grab it from Steam and put it in your library.
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