Chrome: Google's Talk web interface is nice, but annoyingly confined to your Gmail tab. Gtalklet puts a little Gtalk box on any site you want, so you don't have to switch back and forth between tabs just to chat.
Gtalklet is cool, but takes a bit of setting up. After you install the extension, head into the extension's preferences and enter your Google credentials (sadly, Google Apps accounts do not work at this time). Once you've signed in, reload one of your tabs and you should see it pop up in the lower right-hand corner. If not, click the greyed-out Gtalklet icon in your address bar to enable it.
Once you see it in the corner of your screen, just hit the grey circle to set your status. Once you log in, you can set it to away, do not disturb and available. Hit the plus sign to search for a contact and start a new chat. If you don't want it showing up in one of your tabs, just click on the icon in your address bar to hide it. You can even block it from showing up on certain sites from the extension's preferences.
Gtalklet is a free download, works wherever Google Chrome does.