CloudSN (Cloud Services Notifications) is an application similar to Specto (which is not maintained anymore): it can display notifications when you have new emails (POP3 and IMAP), new Identi.ca messages, Google Reader unread items or it can watch any RSS feed for changes.
Psi is a free, open source instant messaging application designed for the Jabber IM network (including Google Talk) compatible with Windows, Linux and Mac OS X.
If you're tired with the Jabber functionality of Empathy or Pidgin, both lacking lots of important features for this protocol, you should give Psi a try.
Gwibber is an open source microblogging framework and desktop client for GNOME developed with Python and GTK+. The Gwibber backend is a stand-alone daemon that manages updates and retrieves stream data from social networks.
Gwibber currently has support for the following services:
Facebook Notify is a small application which monitors your Facebook profile and notifies you when it changes.
It notifies you when any of the following events occur
* One of your friends changes their status, profile picture, or profile details
* You receive a friend request, event or group invitation
The other week, when representatives from Facebook mentioned that they'd be open-sourcing significant portions of their platform, I hazarded a guess that they would be providing at most a set of APIs. Now that Facebook's actually released some code under the aegis of the Facebook Open Platform, I had a look-see. To my delight, I saw more than just APIs.