Litecoin uses a different algorithm for mining blocks, so they are not compatible. However, Litecoin's algorithm is designed to be CPU-restrictive (i.e. doesn't work well on GPUs), so it is possible to run both at the same time. There is a debate about whether Litecoin is all that GPU-unfriendly though, check the alternate currencies forum.
The only cryptocurrencies of any real note are bitcoin, litecoin, namecoin, and solidcoin. Others have crashed and burned, coiledcoin did so spectacularly by including merged mining, so you may not see this feature in future currencies.