MacStories has a good run down:
Cardhop is based on a single text field that sometimes acts as a search field and other times is a text input field. Clicking on Cardhop’s icon in your menu bar opens a detachable drop-down window with a cursor blinking in an empty field. Below that is a column of contact groups on the left, which are the same ones you’d find if you open Apple’s Contacts app, and a list of recently contacted people and upcoming birthdays on the right. If you want to see all your contacts though, they are just one button click away. You can also hide the groups panel with a button at the bottom of the Cardhop window, which reduces Cardhop to its most minimal UI, which is how I prefer to use it because I don’t usually organize contacts into groups.
I think it looks really good, but I don’t have much of a usage for something like this. It sure is pretty though.