The first transfinite ordinal. If you wrote down all the natural numbers on a piece of paper and asked, "what comes next," the answer would be omega-null. It has cardinality aleph-null; however (and here is the part that sucks), so does omega + 1, omega + 2, and so forth. But I'm not a mathematician, and I shouldn't even put that hat on for parlor tricks, so really you should read the writeup below for the gory details.