Appvent Calender has certainly been a successful promotion platform for many iPhone game developers. The latest game to go free is ‘Cabby’ (iTunes Link, Free)

Cabby is a side-on 3D thrust-based game which puts the player in the driving seat of a flying taxi!
Select from 10 different vehicles and fly through 15 unique levels, collecting customers and parcels as you go.
Get your customers to their destinations as quickly as possible to earn a big tip!

Cabby Trailer