Google Talk in Gmail
Originally I thought this was just an interesting novelty, but turns out it’s actually quite useful.
If you haven’t seen it, Gmail now has a bar at the left side of the window, next to your mail, where it will show you your Google Talk contacts and their online status. I've included a snapshot of it.
What it’s really useful for is those situations where XMPP doesn’t work – where the regular Google Talk client can’t talk to the server because there's a firewall in the way. The in-browser version uses AJAX technology so it works great when the standalone version can’t.
This has been done before – MSN’s web-based version of MSN Messenger is a much better IM client than the Gmail based Google Talk interface, and it's available to everyone today. I think the Google Chat interface in Gmail is following the staggered rollout scheme that Google's been using for their new products.