Since testnets do not operate with real money, the incentive to secure the testnets by miners is low. Therefore, the testnets must be differently protected against abuse and attacks. As a result, so-called faucets exist, which distribute free test ether to developers in a controlled manner.
For the Ropsten testnet, different faucets exist:
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http://faucet.ropsten.be:3001/ This faucet provides the possibility to queue the address that should receive the test ether.
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MetaMask MetaMask offers an option to buy ether. If the Ropsten testnet is used, this buy option will allow selecting the Ropsten Test Faucet Service.
The Rinkeby faucet is located at https://faucet.rinkeby.io/. To request test ether it is necessary to make a public post on either Twitter, Goole Plus or Facebook.
The Kovan testnet supports various methods to request test ether. Further information can be found in the Kovan testnet GitHub Repository located at https://github.com/kovan-testnet/faucet/blob/master/README.md.