Xeon Phi (nee Larrabee) looks interesting, but it's not a real product yet. I.e., you can't buy one for your desktop computer.
As others have pointed out, with AMD Opterons you can stuff 64 cores on a server motherboard today.
Well it is about to be available in about 2 months. From Intel: "The Intel Xeon Phi coprocessor 5110P is shipping today with general availability on Jan. 28 with recommended customer price of $2,649."