Flock 1.1 has been released with a small set of nice and important improvements. Most notable integration with Gmail and Yahoo! Mail that lets you know when you have a new message in either service and send links and handle mailto: requests with the service defined as default or you can keep your current desktop application.
Picasa integration also allows to share and access friends and relatives feeds directly from the media bar along with Flickr, Facebook and other photo feeds.
For more details check this recent Flock 1.1 beta review.
Flock is available for Windows, Mac OS X and Linux.