AutoFwd
Had well over 20,000 people use it already and is free.
(Disclaimer my company made it!)
www.AutoFwd.com
e.g. www.AutoFwd.com/angrybirds
It allows you to specify for iOS/Android/Blackberry/Win/Mac/Linux and some others. Also has a default page and statistic tracking with QR generation.
It also allows potential downloaders to enter their email address if their platform isn't supported by your app yet, when you later add the app for that platform they will receive a notification it's been added.
