The web interface of google talk always has this awesome feature called "invisibility" that lets you stay online , but appear as offline to your friends. It is really handy, when you don't want to get disturbed/distracted by someone. But the downside is that the desktop version of gtalk and other IM clients like Pidgin doesn't support this feature.

A Pidgin plugin named GTalk Invisible allows you to set your status as invisible in Pidgin. The plugin is available for both Linux and Windows versions.

The Plugin is available at http://fahhem.com/pidgin/

The installation procedure as mentioned in the site for windows and linux are :

Windows
To install, download and extract the zip file. Move the .dll file (or the one with gears in the icon if you can't see extensions) to "C:\Program Files\Pidgin\plugins\". Restart Pidgin and enable it (Tools->Plugins).

Linux
To install, download and extract the tar.gz file. Move the .so file to ~/.purple/plugins and restart Pidgin.

If have never used Pidgin, I recommend you to try it out because it has lots of feature than you can imagine. Pidgin can be downloaded from www.pidgin.im/