I really like Flycut. Available here: https://github.com/TermiT/Flycut

Like all of the other answers here, it's a clipboard manager which stores every item you copy using ⌘+c in a list. Then you can press ⌘+shift+v to see a list of the items you have copied and choose one to paste. 

Unlike the other answers here, it's free, open source, and doesn't come with a load of other functionality. Also it is actively maintained unlike clipmenu (last release ClipMenu 1.0.0a1 released  Nov 11, 2014).