There are three main test networks supported by the two main Ethereum clients, Geth and Parity. All of them are available for public use, with all of them using their own sandboxed version of the ETH currency. The different networks are not able to communicate together, so each should be considered a completely different platform. It should also be noted that test ether has no real-world value. Though a monetary value for test ether could be created by normal market forces within each network, the fact that any of the networks could be wound down at any time means that this value would be misplaced.