Alternative coins
The Bitcoin software is open source and available to the public. From the public repository, many "alternative coins" have been created by copying the source code and re-configuring specific parameters. Each alt-coin can implement a different ruleset independent of any other alt-coin.
Generally, alt-coins implement different quantities for the total money supply or different block rates. For example, Litecoin has a total supply of 84 million coins and a mining rate of approximately two minutes.
Other alt-coins may implement a built-in inflation. For example, Friecoin has a built-in demurrage that's 5 percent per year. Basically, if you hold 1 Friecoin, it will be worth 5 percent less if you spend it one year later. This was used as an incentive to promote spending.
As another interesting coin, Namecoin uses its blockchain to record information about the ownership of names. The most popular use for this mechanism is to record the ownership of domain names. Using the Namecoin...
