Birchwood Games released Tank Battle some time ago (here's our review) and we found it to be an entertaining game for your Windows Phone. Controls took a little to get used to but overall, a decent battle game on the Windows Phone Marketplace.
Tank Battle is now being offered in a free, ad-supported version. Tank Battle Free has the same battlefield action, same tanks, same game... it's just now has a banner ad running across the top of the screen.
If tanks are your thing, you can find Tank Battle Free here at the Windows Phone Marketplace. If your not a fan of banner ads, the ad-free version is running $1.29 and you can find that version of Tank Battle here at the Marketplace.