Xtify is another webservice for Push Notifications. Similar to Urban Airship, but Xtify seems to position themselves more than just that.
Xtify provides their portal with more control and rules to send out (automated) push notifications. Their slogan is "Mobile Customer Engagement", not just push.
However, sometimes all you want is a simple web service for Push, that is where Urban Airship is more suitable.
The pricing structure for both are fairly similar.