Earlier this week, GMail accounts became enabled to communicate with AOL's Instant Messenger within GMail. It appears most accounts are now enabled. You will have to go into settings and log on to IM to activate the feature. No software necessary. You can connect with your IM 'buddies' now.
Live.com has also implemented a 'sharing' system in recent days where you can see the online status of other Live.com users that you know. We haven't tested it with anything other than @live addresses, but it's probably a safe assumption that the feature will also find Hotmail and MSN accounts.
No doubt Microsoft wants everyone to have an @live address (Hint: Get your name before it's gone!)
So, now that we're all connected, what do we talk about? :)