My favorites are:
The Complete Guide to Microsoft Outlook Add-ins and Utilities
“Microsoft Outlook, the most popular email client around worldwide, includes tons of features that make email easier to use and help us be more productive in our communications. The other great thing about Outlook is that you easily add new features to the software through plug-ins, many of which are available for free on the web.
The Outlook Guide has four parts:
1. Essential Add-Ins for Microsoft Outlook – Whether you want to read PDF attachments inside Outlook or want to make phones calls or need to track email messages, chances are that there already exists an Outlook add-in to help you perform that task effortlessly.
2. Social Networking Add-ins for Outlook – Check out some smart add-ins that bring social networks like Facebook, LinkedIn and Twitter right into your Outlook client.
3. Sync Your Outlook with the Cloud – Whether you want to shift your Outlook data to the online cloud (and vice-versa) or just want to keep your Outlook items in sync with an online service, here are some free tools to help you out.
4. Utilities & Desktop Gadgets for Outlook – Whether you are looking for tools to reduce the size of your Outlook mailbox or need gadgets to access Outlook from the desktop, this guide has a solution.”