iOS (Jailbroken): QuickIM is a jailbreak app that works similar to biteSMS where you can quickly reply to IM conversations without changing apps. This means you can quickly reply to conversations without ever loading up the app itself.
So when you get a new message, you can reply and continue with whatever you're doing. It's simple, but it makes chatting on IM incredibly easy. Unfortunetly, QuickIM only supports Facebook Chat and Google Chat right now, but AIM and Yahoo support is on the way. You can find QuickIM in the Big Boss Repository in Cydia.