AirPush was one company that did (does?) sell ads as Android push notifications. On the whole, they're really not that widely used anymore, though - partly because Android users pushed back hard against them about a year ago, IIRC, but also because the newest version or two of Android makes it really easy to identify the offending app and disable the notifications.
If I remember correctly, the pushback was before the new notification tray, so it may have been Android's own response to the problem.
It is now against the Play store developer TOS to have notifications unless they clearly identify what App it is coming from, I think this makes things like AirPush much more difficult to pull off.
You know, I'd noticed that you can easily find out what apps generate notifications but it literally had never even occurred to me that that was the reason for that addition.
If I remember correctly, the pushback was before the new notification tray, so it may have been Android's own response to the problem.