Crypto currencies

Digital currency, among its various names, is electronic money that acts as alternative currency. Currently, alternative digital currencies are not produced by government-endorsed central banks nor necessarily backed by national currency.

Major Cryptocurrencies
Bitcoin     BTC
Litecoin     LTC
Namecoin     NMC
PPCoin     PPC


Here is a good guide for crypto currency mining.

http://cryptocur.com/litecoin/litecoin-ltc-gpu-mining/