In the world of free games, cross promotion is a key marketing tool.

The latest example of how well it can work comes from the ever innovative chaps at Appy Entertainment.

Its Tune Runner game has been out for just over a week.

As well as being free, it's a viral game where you play tracks in your iTunes library, posting competitive online scores depending on your performance.

This means the more people playing, the wider the variety of tracks played and the more frequently they're played, which in turn means the more people play. Hence Appy wants to get Tune Runner out everywhere.

How to do this?

Well, with its previous game FaceFighter, a #1 game during 2009 (which comes in paid and free flavours), the obvious thing to do was to cross-promote Tune Runner in FaceFighter.

"We set FaceFighter to free for the weekend, and it looks like we shifted over 700,000 downloads," explains an excited Paul O'Connor, Appy's brand director.

But what happened to Tune Runner?

It's reached #3 in the free music game category in the US, UK, and Canada, trailing only the free versions of Rock Band and Tap Tap Revenge.

It's also entered the free top 50 games chart of 11 countries, including the US, UK, Japan, Canada, France and Germany

Or as O'Connor puts it; "We've taken a new and original music game IP from nowhere to near the top of the world market in just over a week."

You can read more about the process, and what's planned next for Tune Runner, on the Appy blog.