iPhone OS: Boxcar is a push notification app for iPhones and iPads that works with your email account(s), Twitter, Facebook, RSS feeds and more. It's one app, and a whole lot of possibilities for instant alerts that'll keep you informed on-the-go.
Boxcar isn't a new app, but it did just launch a free, ad-supported version. After you install it, head to Settings -> Add and Edit Services to set up push notifications for the various services you'd like push notifications for. The Twitter and Facebook setup is pretty straightforward, but the email is a bit different. To set up push notifications for your email, you'll need to set up a filter in your email account to forward messages you want pushed to your phone to a unique Boxcar email address. That means that in Gmail, for example, you could set up pretty specific filters so you're only notified of the most important messages.
You can also set preferred actions for what Boxcar will do when you receive notifications. For example, Boxcar can open one of almost 30 different Twitter clients; if you've got a Gmail account, it can open straight to the web interface. Or, if you don't like being redirected after a notification, you can simply choose to do nothing.
Push notifications are one of the best features of the iPhone OS, and Boxcar easily adds push to the services you're most likely interested in receiving push notifications for. Even better: It's now a free download for iPhones, iPads and iPod touches.