Chrome: Google's chat feature is useful for sending quick IMs and starting video chat sessions, but you can only view it when you're on a Google page like Gmail. Google has released an official Chrome extension that detaches it from your browser, allowing you to view it no matter what web page you're viewing.
This extension has actually been out for a while, but was Chrome OS only. Now, you can grab it for Chrome on other platforms, letting you carry on chats, make voice calls, and even start Google Hangouts right from your desktop (my personal favourite feature). It's very helpful if you're tired of switching back to your Gmail tab to chat. Hit the link to check it out.
Chat for Google is a free download, works wherever Google Chrome does.
Chat for Google [Chrome Web Store]