Most of the software/service vendors that I deal with are quite good, but every now and then I have an exceptionally positive experience that makes it worth mentioning to others. This one solves a very specific problem, but one that a lot of people face, and the developer's responsiveness to support requests has been absolutely superb.

NK2Edit address an issue that has plagued Microsoft Outlook for many years. The auto-complete feature, which is supposed to remember names and addresses of email recipients and fill them in as you type, is notoriously unreliable. The file where this cache of addresses gets stored frequently becomes corrupted, fails to preserve recent changes, or simply disappears for no apparent reason. NK2Edit allows this data to be viewed, edited, repaired, backed up and restored, and continuously monitored, for a single user or for every user on your network, all in a simple, easy-to-use interface.

Free for personal use, and very reasonably priced for commercial use. Highly recommended.