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 ticket fees and the artists that it works with. If there's an alternative way to buy your tickets, either from the venue itself or from the artists involved, it'll probably be cheaper than Live Nation, and better for the industry at large.
But they are a huge company with a lot of partnerships going, and quite a few shows nowadays (if not all of the big venue shows) are sold through Live Nation. So if you're planning to buy tickets with them anyway -- and sometimes you don't have a choice -- using the app will give you some nice bonuses.