Codemasters’ original Grid title (sold in some regions as Race Driver: Grid) is free through the Humble Store for the next couple of days. As tends to be the case with this type of giveaway, you’ll need a Humble account in order to claim the key.
It’s a Steam key too, so you’ll need a Steam account. Chances are pretty high that you have one of those, though. Looking at the timer, it seems like Grid will be free to claim until around 10am Pacific on 24 December. You’ve basically got (at the time of writing) two full days to get a code.
The game has you competing in various driving disciplines (muscle car, drifting, circuit) in a laundry list of international locations like Milan, Tokyo and San Francisco. It was released in 2008.