As part of the Tenth Anniversary special for Battlefield, they are giving away Battlefield 1942 for free!
That's right, the original one! It has full mod support, so you can put it on, and then add the desert combat mod so you can have the most intense helicopter flights while killing legions of bots at the same time.
Again, this is free - you click the link below, then launch your origin client to add it to your games list.
It is about 1.2 Gigabytes in size.
Link to the store with 1942
Click above, then add 1942

Although it may mention it's in the demo category, it is the full game. Yipee!
Desert combat you say, whats that you also say?
Well, it was a modification of the game originally made to make the game be in modern times. The team was eventually hired and helped out with Battlefield 2.
Image with thanks to IGN.
You can download the Desert combat mod by following this link, dated 2004! Remember you'll need to download the 0.7 client as well there.
Have fun!