Game Channel is a free downloadable iPhone/iPod Touch app. In the app store, it was originally called just "OpenFeint", then it was called "OpenFeint Game Spotlight", and now it is called "Game Channel : Free Games from OpenFeint". The app is developed and published by Aurora Feint Inc., and is fully supported by the Bloons TD game app. The current version of the OpenFeint app is 2.2.1 (App Store Post Date: March 30, 2011). It was originally a 2.1 MB download, and it is now a 5.4 MB download. As of June 18, 2011 the app is rated just under 4.5 stars in the app store.
OpenFeint provides achievements, leaderboards, forums & chat, friends, challenges between friends, and fan clubs for games that support it. Not every game has every OpenFeint feature, but Bloons TD supports leaderboards, achievements, challenges, forums & chat, fan club, and a "Who's Playing" of your friends.
For a list of the OpenFeint achievements for Bloons TD, click here: Achievements
OpenFeint can be accessed in-game from the main menu by tapping "Play Game" then tapping the OpenFeint leaf icon in the top right corner (see picture below).