RSS.app is a very simple Mac app developed by Joost Schuttelaar following the removal of RSS feeds from Mail and Safari in Mountain Lion. RSS.app sits in the menubar, and checks for updates to your feeds every few minutes or hours. It then displays new article alerts using Notification Center.
The app is almost invisible: It is embedded in the status menu and uses Mountain Lion's Notification Center to alert you of new posts. You can use RSS.app to import your existing list of RSS feeds from Apple Mail — even after you've updated to Mountain Lion.
The list of feeds to check can be configured in the app's preferences, and you can import feeds from Mail even if you've already upgraded to Mountain Lion. The app is extremely simple: it's got no keyboard shortcuts, menus, or distracting interface elements -- it's just a menubar checker for RSS feeds. Clicking on a notification will take you to the article using your default browser, and that's it.
As Michael Tsai noted, the app has been rejected by Apple because of rule 2.8, which says "Apps that are not very useful or do not provide any lasting entertainment value may be rejected".
If you think RSS.app can be useful to you, download it here.