iClip

After being invited to beta test iClip years ago, it quickly became one of my most-used apps. There are a lot of clipboard solutions out there (I've tried at least 25-30), but this one gets the most things right. iClip is unobtrusive, ultra-customizable, lightweight, reliable, and is easy-to-use. And the developer (Irradiated Software) is experienced, credible, responsive, and has an impressive portfolio of Mac utility apps (such as Tuck, Cinch, TotalFinder).