Linking to the App Store from directly within your application is a great way to let people know about your other apps or to upgrade from a free version to a paid version. Doing this is actually quite simple. Just put a button in your app and link the action to the following code:

-(void)purchaseFullVersion {
    NSURL *url = [NSURL URLWithString:@"itms-apps://itunes.com/apps/appname"];
    [[UIApplication sharedApplication] openURL:url];
}

If you want to link to all of your apps:

-(void)viewAllApps {
    NSURL *url = [NSURL URLWithString:@"itms-apps://itunes.com/apps/developername"];
    [[UIApplication sharedApplication] openURL:url];
}