One of the utilities that I wonder how I ever got along without is called Jumpcut by Steve Cook.
My day job is working for an internet company so I live in my browser for hours and hours everyday. I am constantly copying or cutting and then pasting a piece of information from one place to another or more likely more than one piece of information. For example, before Jumpcut if I had 3 pieces of information to copy from someplace into a form I would have to do this one piece of information at a time, often switching windows or applications in between. Jumpcut lets me paste in any of the last 25 things that I cut or copied into the clipboard. Instead of doing a paste with command-V, I use control-option-V to pop up a transparent overlay that shows the current clipboard. If I hit the arrow keys while continuing to hold down the control and option keys then I can scroll through any of the last 25 clipboards. When I am done, I have pasted the information and also changed the current contents of the clipboard.
The following video shows how Jumpcut works:
Jumpcut is free. Get your copy here.