How to integrate Skype into Pidgin?

You can install the pidgin-skype package from the Software Centre (or using apt-get)

This protocol plug-in allows libpurple to communicate with Skype. Applications using libpurple (Pidgin, Finch, Empathy/Telepathy, etc.) can thus show your Skype contacts alongside those from other protocols, and you can communicate with them using that application instead of the Skype user interface.

This plug-in communicates with the Skype application in the background to perform its work, so it's necessary to have Skype installed and running. This product uses the Skype API but is not endorsed, certified or otherwise approved in any way by Skype.

That previously recommended answer is out-of-date. The new version of the plugin (by same author) no longer requires microsoft's skype client running in the background. It also has a new name, pidgin-skypeweb.

pidgin-skypeweb can be installed like this, from a 3rd party PPA:

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys C03AA79CFFEBD240 \
 && echo "deb http://ppa.launchpad.net/aap/pidgin/ubuntu wily main" >> /etc/apt/sources.list \
 && apt-get update && apt-get install -y pidgin-skypeweb

Skype Plugin for Pidgin

This is a Skype Plugin for Pidgin/libpurple/Adium. It lets you view and chat with all your Skype buddies from within Pidgin/Adium. You still need Skype to be running to be able to use it, but it lets you keep a consistent user interface and use all the other nifty Pidgin/Adium plugins with it, like spell-checking or OTR encryption.