iOS and Android: Steam's desktop apps for Mac and PC are fantastic not just for playing games, but communicating via chat, organising via Steam Groups and even reading news on upcoming titles. The new iPhone and Android apps let you do all this on your phone.
The apps are downloadable right now from their respective stores/marketplaces. Logging in and typing in your access code (it gets sent to the email on your account) will put you on the beta list.
Once you're in, you can see and chat with your friends list, with all the messages being mirrored on your phone and on your desktop, if both are on. This way you can have a chat log waiting for you on your computer if you're out. Unfortunately you don't actually get push alerts outside of the app when someone talks to you (at least, I don't), which makes this not as useful as it could be. Definitely a feature Valve needs to add.
Beyond chats, you can also look at your friends' activity log (what games they bought, achievements they got), look at your Community Groups' announcements and messages (but you can't post), and browse the store for things to add to your wishlist. What's also neat is that you can actually make purchases from the phone itself, so if you can't get to a computer during a Steam sale, you can make sure you don't miss out.
Once Valve fixes these little bugs and adds the missing features, the Steam app for mobile will be a fantastic assistant for people who purchase and play games regularly on Steam.