This plugin provides you with an ingame instant messenger. This can be used with numerous services, including GoogleTalk access.
The plugin can also be used to easily initiate voice chat or coop games (including NAT hole punching) with your contacts - assuming they're also using the plugin (the linux version of pidgin also supports compatible voice chat).
Note: google suck, if you want to use a gmail account then you will need to disable 2-factor auth first via https://myaccount.google.com/intro/security
Note: google requires that your password is stored locally, and unsalted. This will be stored inside an 'xmpp.cfg' file inside your basedir. This same file allows the plugin to automatically sign in next time you start the game.