Add Twitter support to Microsoft Outlook 2003, 2007, 2010 with TwInbox
TwInbox began life as OutTwit a couple years back, and it's steadily improved over that time. The current version is a 490kb add-in that works in Outlook 2003, 2007, and even the 2010 Technical Preview (if you hadn't guessed from the screenshot).
The usual Twitter client features are included. Highlight a message and you can retweet,or send a reply or direct message to the sender. URLs can be automatically shortened using TinyURL, and TwInbox can even handle uploading pictures and other file attachments. Searches are also supported, allowing you to receive updates for certain keywords from users you aren't following.
Perhaps best of all, tweets behave just like any email messages once they have arrived. That means, of course, they are fully searchable and you can also utilize Outlook's and use the rules wizard to organize, filter, and prioritize your stream. Want to share a tweet with co-workers? Highlight and forward it as you would with a standard email.
If your company uses Twitter for customer relations and happens to run Outlook as well, TwInbox is a great way to centralize communications.
Apart from all the practical applications there are for TwInbox, it's also a fabulous way to discreetly tweet from your cubicle at work. Not that any of you have neighbors with wandering eyes...