Google Talk is pretty boring. It's an instant messaging service, but there are a ton of others out there. Gmail lets you use it to message other users as you check your mail, but I can do the same thing with my Facebook
account. Voice chat was only available for PC users, and it wasn't as powerful as cross-platform Skype
Mac and PC users will both get to enjoy video chats from within Gmail within the next couple of days. You'll have to download a client the first time you use it, but after that you can video chat with your webcam or just voice chat with a microphone. I can't wait to try it out.