I like the new Windows Live Messenger app. It's pretty basic, but all I need is IM and it delivers pretty well. The deal-maker for me is the Hotmail integration, I can't use Main with my Hotmail account because I literally have 6900+ emails and the iPhone tries to load ALL of them, and I don't have data so it's not super useful. Plus, WLM for iPhone is free so that's a huge bonus.