Although Bitcoin is seen as the first viable digital currency, many alternative digital currency projects, also known as altcoins have since followed suit. Creating an altcoin is quite simple, due to the open source nature of Bitcoin and other similar projects, a developer may simply copy the project, make some minor adjustments to the source code, and launch his new coin.
Most altcoins operate under the premise that they differentiate in some way from Bitcoin. Some focus on other types of utility, others seek to redress real or imagined problems in the Bitcoin design.
ASICs are seen as a centralizing force by many, and an obstacle to a marketing mechanism of Bitcoin in which new users may "print their own money" through mining. Various proposed technical solutions to ASIC mining have prompted dedicated altcoins, however these solutions so far are based on invalid assumptions, all have been proven unviable in theory, some have been proven unviable in practice as well.
Proof of Stake
The core concept of Bitcoin's key innovation: proof of work, relies upon a mechanism of provably spending energy. Some criticize this entire technique and propose that another mechanism be found, the principle proposal being: proof of stake. In Proof of Stake, amount of held coins translates to the mining power equivalent. However this concept's flaws prevent it from working: nothing at stake attacks, hard fork resolution uncertainty, and other problems prevent these altcoins from passing muster.