Is there an altcoin that is only feasible to mine by CPU? I'm interested in mining altcoins, mainly to turn some servers into (literal) space heaters, since it's winter. One thing that really interests me is if an altcoin has been designed to only be mineable by traditional CPUs. The big thing I can think of to cause this is large memory requirements. A large memory requirement for hash processing would cause current GPUs to be slower than CPUs (due to the individual "cores" of the GPU not having enough direct memory). It would also cause ASICs and FPGAs to be very difficult to develop to be faster than CPUs, and expensive. This is because you effectively reduce the mining complexity so that it bottlenecks at the memory bus (which is much harder to significantly speed up)

I'm sure I'm not the only one to have thought of such a scheme. Is there an altcoin out there that is designed with this kind of ideal behind mining?
Litecoin is the 2nd most popular currency in the same family as Bitcoin, and:


  It is based on the Bitcoin protocol but differs from Bitcoin in that
  it can be efficiently mined with consumer-grade hardware. Litecoin
  provides faster transaction confirmations (2.5 minutes on average) and
  uses a memory-hard, scrypt-based mining proof-of-work algorithm to
  target the regular computers with GPUs most people already have.


https://litecoin.org/

The intention was to allow CPU mining to be viable, but according to Wikipedia it hasn't quite met this goal:


  Litecoin uses scrypt in its proof-of-work algorithm: a sequential
  memory-hard function first conceived by Colin Percival.[14] The
  original intended benefit of using scrypt was to avoid giving
  advantage to GPU, FPGA and ASIC miners over CPU miners, which occurs
  in the Bitcoin protocol. However, this turned out to be an incorrect
  assumption: GPU mining in Litecoin's implementation of scrypt is
  currently ten times more efficient than CPU mining


http://en.wikipedia.org/wiki/Litecoin