Facebook Desktop is a free and simple tool that allows you to check all events and notifications that you receive in your facebook account without having to open the explorer and navigate to facebook website.
Among the notifications you can receive, unread messages, pokes, friend requests, wallposts or any other third party application you may have configured in your facebook.
Security isn´t a concern, most ebay or facebook applications request you to input your password within the application, this can be considered a security thread; however Facebook Desktop redirects you to Facebook website to request your permision, therefore it never capture your user name and password.
Facebook Desktop have an IM like interface, which will display your picture and latest events, no more, this can be poor for some users that might like to use it like a Windows Live Messenger, but for me is ok.
If you like Facebook.com and are not using any gadget application, Facebook Desktop will save you the loging process each time you need to check your account.