The most famous chatting application by Google, Gtalk is facing a service issue since last 2 hours. Many people around the world are not able to log in to the service. Those who have logged in, not seeing any contacts, neither able to start chat by invitation.
About the issue, Google updated on the AppStatus portal
"We're aware of a problem with Google Talk affecting a majority of users. The affected users are able to access Google Talk, but are seeing error messages and/or other unexpected behavior. We will provide an update by 7/26/12 5:20 PM detailing when we expect to resolve the problem. Please note that this resolution time is an estimate and may change."
Even at the time of writing my post, it was not clear when to expect the service up again.
Service Monitoring
Google has a dedicated page for providing service information about its various Apps. You may subscribe to their feed for being updated about their various service disruption and outages.