I'm obsessed with Paste 2. It's SO GOOD. I used flycut before this, but it doesn't compare. The UI is really great. Saves different types of content (text or images) and you can quickly search for old snippets without taking your hands off the keyboard.
I use this constantly. I agree with Mike that the the UI is great. The shortcuts are lifesavers and you can organize things in your clipboard into categories. The only thing I wish is that you could lock clipboard entries — sometimes I accidentally delete things.