We're big fans of Google Chat, which allows us to chat with other Gmail users from within the Gmail interface. Since we virtually always have Gmail open, this works out pretty well. If, however, you find yourself wanting to access Google Chat without necessarily opening Gmail, Gtalk is a pretty good option. This Chrome plug-in allows you to use both Google Chat and Google Talk in their own little messenger window, totally independent of Gmail.
Gtalk appears as an icon to the right of Chrome's address bar, and clicking on it opens a drop-down menu, which is odd, given that Google Talk is the only option; why require an extra click? This is a fairly minor complaint, though. The messenger window opens and lists available contacts; begin a conversation with one and a new tab opens within the messenger window. The program also supports voice chat using Google Talk, but users must first download the separate Google Talk client. Gtalk's integration with the Google Talk client is a little questionable; sometimes clicking the Call button in Gtalk launched Google Talk and initiated a call, and sometimes it brought up an error message. Other than that, though, the Gtalk extension worked just fine, and we found it to be a lightweight, useful way to access Google Chat and Google Voice. It doesn't have any mind-blowing features, but it covers the basics quite well.
Gtalk is free. It installs and uninstalls without issues. We recommend this add-on to all users.