In case you don't know, Gtalk is Google's IM tool which you can use with your Google account as login. Gtalk was conveniently embedded in Gmail a while ago, so that you can chat with your friends while checking the email.
Now, gTalk Sidebar embeds Gtalk into the browser so that you can keep on chatting even if you close the Gmail webpage. This Firefox extension displays Gtalk in the browser's sidebar, where you can see your contact list and start chatting right away.
What's more, you can easily jump between different chat windows thanks to tab support. The program also includes a nice set of emoticons.
gTalk Sidebar can also be used in a pop-up window by clicking on the "Pop out" link, although if you do that in the middle of a conversation it won't show up in the pop-up window.
Also, another weird behaviour in gTalk Sidebar is the fact that it opens a new browser window for you to log into your Google account, instead of letting you log from the sidebar itself.