Libjingle is a set of components provided by Google to interoperate with Google Talk's peer-to-peer and voice calling capabilities. The package includes source code for Google's implementation of Jingle and Jingle-Audio, two proposed extensions to the XMPP standard that are currently available in experimental draft form. Below is a summary of the individual components of the library. You can use any or all of these components. base - Low-level portable utility functions. p2p - The p2p stack, including base p2p functionality and client hooks into XMPP. session - Phone call signaling. third_party - Non-Google components required for some functionality. xmllite - XML parser. xmpp - XMPP engine. More about Google Talk Libjingle
Hopefully, we'll see voice and video capabilities in GAIM soon (2.0beta doesn't have any yet, although Sean Egan announced it some time before). Once GAIM/GTalk compatibility will hit the market, I think they'll gain a lot.