There are a lot of great Twitter clients for the iPhone, but whether your favorite is Tweetie, Twitterrific, Birdfeed or something else, they're all missing a feature that I find really useful. I'm talking about push notifications, and even the top-shelf Twitter clients don't support them. Although that's likely to change when Twitter finds a workable push solution for developers, an iPhone app called Boxcar has stepped in to fill the gap.

Boxcar is a 3-dollar app that polls Twitter for new direct messages and mentions, and notifies you via push when you get one. It doesn't replace your current Twitter client, it actually opens your @s and DMs using Tweetie or Twitterrific. Other clients may be supported soon, but if you don't use one of those two apps already, you should hold off on Boxcar for now. If you do, it might be 3 dollars well spent for you.

Boxcar is not without competition, though. For folks who want Growl notifications pushed from their desktop to an iPhone, whether from a Twitter app, a mail client, or something else, it's worth taking a look at Prowl. If you're willing to leave a Mac Twitter client running while you're out and about with your iPhone, you can achieve the same effect as Boxcar, with a few extra frills.