Prpltwtr is a Pidgin (or Empathy) plugin which makes Twitter feel a lot like an IM: you can see your followers in the list (this can be disabled and is recommended to disable it if you have a lot of followers) as well as saved searches and so on. Direct messages will open in new conversation tabs, you can reply, retweet and much more!
There is already a plugin to use Twitter in Pidgin - MBPurple, but I find Prpltwtr to be far superior because it really makes Twitter feel like a IM protocol.
The developer also lists Identi.ca and Status.net under the supported microblogging protocols, however I couldn't figure out how to add my Identi.ca account - if you find how to do it, please let us know in the comments (Update: after compiling it again, Identi.ca and Status.Net showed up).
Prpltwtr works with Empathy too but I've only tested it with Pidgin and the installation instructions below will be for Pidgin only. For Empathy, see THESE installation instructions.
Installing Prpltwtr in Pidgin (Ubuntu) is very easy, all you have to do is download the source files from HERE, extract it, navigate to its folder using the terminal and then:
sudo apt-get install libpurple-dev
make && sudo make install
Once the installation is over, enable Prpltwtr in the Pidgin Plugins dialog, then add your account just like you add any IM account in Pidgin. On the "Modify Account" window, click the "Advanced" tab to see all the available options for the Twitter protocol.
You can also use Prpltwtr in Windows - see it's download page.