What's the best "iMessage" for Microsoft?

What would be the best iMessage type software for Microsoft? Please don't say MSN messenger. That's so retro it makes my stomach ache. One of my brothers is a hardcore Apple user. My other brother and I are big time Surface Pro users. While I like apple products, and particularly the iMessage feature, I don't use the products for work such as an ipad. And I wish I had a nice app where I could speak to my brother and friends with a surface or windows product that would connect such as imessage does.