All apps featured in this post are free.
The Kentucky Derby and Breeders' Cup apps haven't, of course, been updated yet for 2012, but both appear like they'll be very useful when the time comes. Don't forget to check them out during the spring and fall respectively.
It's also worth noting that while there is no TDN app, all features on the TDN website are fully functional with Safari. The newsletter itself looks great on the iPad either in Safari or in iBooks, and video replays are compatible with iOS.