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The MultiCoin client is a slightly modified and very experimental Bitcoin client that allows transactions with an unlimited number of crypto currencies block chains and also the creation of new crypto chains. Each different crypto currency can have a different set of rules and a different root block that is controlled by settings in the MultiCoin config file.
Tenebrix was built using Multicoin. Tenebrix was later cloned by coblee whose Fairbrix was the prototype of his more successful project, Litecoin.
Seemed like a pretty interesting idea. I wonder what happened to it?