MyPSN

Designed as an unofficial desktop client for My Playstation, MyPSN is a tool that lets you connect to your PS account and stay in touch with your friends.
It provides access to the same set of features as the official web service. The key difference is that all those actions can be made from the comfort of your desktop, so you don't have to rely on a web browser anymore.
Following a speedy setup operation that shouldn't give you any headaches, you can get started by entering your My Playstation credentials after reaching the main app window, in order to link the desktop app to your account.
Worth noting is that the app cannot automatically identify your login credentials if you are already connected in the web browser. Next, you can check out your profile, friends and trophies, just as you normally can when using the My Playstation web service.
Furthermore, you can add new friends and manage friend requests, as well as receive notifications. Some actions are not permitted, through, like editing your profile or managing privacy settings.
On top of that, the main window cannot be resized or even sent to the taskbar, taking up the entire screen until it's closed. To be fair, the software project is fairly new so there's plenty of room for improvements.