For a while I have had my Twitter account set to send SMS messages to my phone whenever I received a direct message. It is a nice way to be alerted quickly to direct messages sent from folks I follow on Twitter. Of course every time I got one, I was charged for it by my phone service provider. (as I write this I'm wondering why the various Twitter clients on the iPhone don't already do this?)
I recently read about an iPhone app called Boxcar that allows you to set up push notifications on your iPhone for your Twitter account. Push notifications are messages that come to the iPhone as they occur, much like a SMS text message. Setting up Twitter notifications with Boxcar was pretty simple and it works as advertised. Boxcar also advertises that it can provides push notifications for RSS feeds also. So far I have not been able to get this feature to work and from reading their support forums, it seems as if this feature is not yet working.
This lead me to another application called Notifications. It provides push notifications for RSS feeds, Gmail, and Google Voice. For example I have set Notifications up to alert me when we publish new content to our school web sites. When new content is published the RSS feed is updated and a notification is sent to my iPhone.
PushMail is another app that allows you to set up push notifications for your gmail account. Using Gmail's filtering tools, you can set up PushMail to alert you to specific types of email, or email from specific addresses. For example at school we have set up a Google Form for staff to enter information about interactions or incidents involving students. The idea is to have a place where staff can share this information with me and other support staff so as to make everyone more aware of specific issues or incidents with students. We use this as a way to track these throughout the building. I would like to be alerted when a new entry has been made to this form, and the Google form allows me to send myself an email notification whenever the form is updated. By using PushMail, I now get these notifications as they are entered.