I enjoy being able to seamlessly move between all of my devices and have Facebook Messenger everywhere - with it I can chat cross-platform with ease. But when I'm on a Mac, I was always annoyed at having to find a browser or remember to keep it open - until I discovered Astro for Facebook Messenger. It's a free OS X app available in the Mac App Store.
It displays all messages from all conversations, no matter where you send the message from including the web browser and mobile. You can change the skin on the app, and even drag and drop photos. But I think my favorite feature is the nice menu notification bar - I can see at the top of my OS X menu that I have unread messages, and the app pops to the front just by clicking on the icon!