If you're a fan of Outlook.com but still use a few other accounts for mail -- like Gmail or Yahoo -- you can import your mail and manage it from Outlook.com's awesome interface.
Gmail importing has actually been around for a while, but now Outlook supports most other IMAP-enabled email providers. To do so, just head to the Import Email Accounts section of Outlook's options and enter your credentials. Not only will it keep your email in one place, but you get to use Outlook's great interface to manage it all.
Note that this is not to be confused with the IMAP support which was added in September, which lets you access your Outlook inbox from desktop apps like Thunderbird. Essentially, this lets you use Outlook as a mail client for your other, non-Outlook accounts. Hit the link to read more.