If I have got three add-in to recommend for Outlook these are the one below. Outlook is maybe not one of the best e-mail client, but this is certainly  one of the most common client as it goes about business mail client. The tool can be perfected though with the addition of some well chosen add-in. These should make your life easier while getting helping you to get rid of the clutter that your Inbox may suffer from.

• Xobni: quickly retrieve your e-mail quickly and enrich your contact details with pictures taken from Facebook and LinkedIn. Xobni also performs some very insightful datamining about the e-mails you send and receive such as your most e-mailed correspondent [Free and Commercial]
• ClearContext: a good addition to Xobni as it prioritizes your mail and adds some useful features such e-mail follow-up [Commercial]
• EzDetach and Mapilab Attachment processor: It's hard to chose among the two, but the first may be simpler to use than the latest, while serving the same purpose, remove the attachment and store them in a safe location while replacing them by a link to the corresponding file [both Commercial]
• Mapilab e-mail follow-up: Though redundant with one of the feature of ClearContext, this tool may be a good choice as a standalone add-in for ensuring your e-mails are followed-up by their addressee, and if not to send them a reminder in due time 

Have also a look to this Xobni demo:

And this is for ClearContext: