Since you can use Outlook to manage your contacts, calendar, and e-mail, why not use it for Twitter as well? That's the idea behind TwInbox, a free plug-in that adds a Twitter toolbar to Outlook.
Once installed, TwInbox adds a new folder to your inbox. (It can also create individual folders for each sender, which can be helpful from an organizational standpoint--unless you follow lots of people.) From there you just supply your Twitter username and password, and presto: It fetches all the tweets from your account and lets you read them just like you read e-mail.
A click of the toolbar is all it takes to send out your own update, send a direct message, reply to a message, or retweet something. TwInbox cleverly auto-shortens long URLs via TinyURL. The toolbar also shows you a preview of new tweets as they arrive, which saves you having to switch to the tweets folder every time you want to check for updates.
In short, TwInbox is a terrific little tool, and a must-have for any Twitter-loving Outlook users.
Note: This program does not work with Outlook Express.