Montony is a free app which notifies you on RSS feed updates. Unless other RSS reader apps, it is a fine and portable app which can help you keep updated by subscribing to your favorite blogs.
Montony is very tiny app, just 800 kilobytes which sits in your Mac finder bar if you want, though you can disable it too if desired. It uses Growl like notifications at the top right of your screen to display the feed update notifications, which includes the Feed Title, the blog name and little bit of excerpt of the feed.
You can set in preferences whether you want Montony to run at launch of your Mac. You can use let the app use Growl for displaying the notifications. Monotony integrated with Growl to display customized Growl style notifications which you can customize further by changing the preference in Growl.
There is also "Run Invisible" option, when it is switched On, the Montony notification icon doesn't displays on the finder bar, and when it is switched off, you need to open the preference right from the app in the Applications folder.
Montony for Mac [Free]