In Jelly Bean, Google introduced an incredibly useful new feature that allows users to find the source of a notification by long-pressing on it and selecting "app info." This is fantastic for those occasions when notifications are showing up without a clear idea of what is causing it. The problem is, that's only helpful on devices running Jelly Bean (which isn't a whole lot at this point).
For everyone else, there's a new app called Notification History.
NH is very simple: it runs as a service, collecting your notifications. Thus, you can go back later and not only check to see which apps are the cause of annoying notifications, but also read or re-read ones that have already been dismissed. If that wasn't enough, NH also allows you to backup and share notifications (though I'm not entirely sure why you'd want to share a notification).
Either way, the app is completely free, so hit the widget below to grab it.