Gmail Chat added a feature that was initially released in the Google Talk gadget: playing videos from YouTube and Google Video without opening a new page. If you paste a link to a video from YouTube or Google Video in a conversation, Google shows a clickable thumbnail that can trigger an inline video player.
If you tried the audio/video chat plug-in for Gmail and it didn't work well, there's a chance that the latest updates solved your issues: incompatibility with Creative camera drivers, errors when sending and receiving video for CPUs without the SSE2 extensions, lack of support for many USB cameras on a Mac OSX.
{ via Gmail Blog }