Social Lite is a nice tool for managing three popular sites. The app can connect to Facebook, Twitter and Gmail, all from the same interface, in a single window. It runs on your system's menu bar, from where you can access it by clicking on its icon. When you do that, a small window will pop up (down rather) where you can browse those sites. The window shows three buttons at the top: one for Facebook, one for Twitter and one for Gmail. After you log in, you will see the mobile version of these sites and you can do anything that you can do from the mobile version. There are certain limitations, at least on Facebook. For example, there is no chat, you can't upload pictures, etc.
There are notifications, but not the type that you think. You won't get a pop-up when you get a new e-mail or Facebook notification. New e-mails will show when you refresh the page or the app does it once every 5 minutes or so (you can customize this). For Facebook, you will get notifications at the top of the screen.
All in all, this is a great way of managing your three accounts from a single location. It has some limitations, but some users might not care about those. The app is very popular on the Mac App Store. The Pro version has instant messaging and it shows a number with unread notifications on its menu bar icon.