Boxcar: Real-time Twitter push app for iPhone

Boxcar is a 3-dollar iPhone app that sends you push notifications anytime someone mentions you on Twitter, or sends you a direct message. The best part is, it works with what you already love: Tweetie or Twitterrific. It check your Twitter account, every 2 minutes for new @mentions and direct messages. If there is anything new, a push notification is sent to you!