Ticket seller Live Nation has released its very own iOS app on the App Store -- you can download it for free there right now. Not only can you browse upcoming events and buy tickets to shows directly from within the app (as well as get news about presales and upcoming deals on tickets), but once you go to a show in person, you can pick up setlist information, see photos and videos and check-in with friends through social networks.
Personally, I have kind of an issue with Live Nation, given its rocky past with Clear Channel Communications and Ticketmaster, so I can't really recommend the app just to buy tickets with. The company's never really been generous to customers, to say the least, taking advantage of high tick
