To communicate with your family, friends, and co-workers, the two apps to use are Mail and Messaging. Now, it'll be foolhardy for Microsoft to allow integration with only its own services - thankfully, that's not the case. In Mail, besides Hotmail and Outlook (including Exchange, of course), you can also add Gmail, AOL, and Yahoo! Mail accounts. We like that messages from different parties in the same email thread are grouped together so that it's easy to follow the conversation. You can even pin email accounts to the Start screen so that you can see new mail on the live tile without opening the Mail app. You can also configure accounts from other email providers if you know things like the account type and incoming and outgoing email server settings.
On the other hand, Messaging supports Microsoft Messenger and Facebook. Sorely missing is support for Google Talk and Twitter, which is strange considering both Google and Twitter accounts are supported in the People app (more on that later).
The Calendar app supports Hotmail, Outlook, and Google accounts, and is able to combine information from multiple accounts, and present them in either a single day, week, or month view. It also supports live tile updates, so you can see upcoming events from the Start screen. You can even choose to see upcoming appointments on the lock screen.