As mentioned in this answer, if you have access to an RPC node or geth, the easiest way is to use the builtin eth_sign functionality.

There are various libraries for Javascript which let you do that manually if you are not afraid of writing some code. An example is shown by Niran.

If you have (or are happy using) the node.js environment, you can try out helpeth which is a command line tool for managing keys and signing:

$ helpeth --password 'Use --password-prompt instead for security' --keyfile UTC--2016-03-17T19-06-57.064Z--15f2f3e0f2d74ea7b185fc12f24cb4f402cc96d0 signMessage 'Hello World'
Input message: Hello World
Message hash (Keccak): 0x592fa743889fc7f92ac2a37bb1f5ba1daf2a5c84741ca0e0061d243a2e6707ba
The signature: 0x167760997a69e225c0668e6761cd20cac70f3a6ace29fe2d287c3003daf6972b10d158a47e8f064cf982a3defdf236247c41249dbfb0fb81f0d126c26a94971d01

As mentioned in this answer, if you have access to an RPC node or geth, the easiest way is to use the builtin eth_sign functionality.

There are various libraries for Javascript which let you do that manually if you are not afraid of writing some code. An example is shown by Niran.

If you have (or are happy using) the node.js environment, you can try out helpeth which is a command line tool for managing keys and signing:

$ helpeth --password 'Use --password-prompt instead for security' --keyfile UTC--2016-03-17T19-06-57.064Z--15f2f3e0f2d74ea7b185fc12f24cb4f402cc96d0 signMessage 'Hello World'
Input message: Hello World
Message hash (Keccak): 0x592fa743889fc7f92ac2a37bb1f5ba1daf2a5c84741ca0e0061d243a2e6707ba
The signature: 0x167760997a69e225c0668e6761cd20cac70f3a6ace29fe2d287c3003daf6972b10d158a47e8f064cf982a3defdf236247c41249dbfb0fb81f0d126c26a94971d01

As mentioned in this answer, if you have access to an RPC node or geth, the easiest way is to use the builtin eth_sign functionality.

There are various libraries for Javascript which let you do that manually if you are not afraid of writing some code. An example is shown by Niran.

If you have (or are happy using) the node.js environment, you can try out helpeth which is a command line tool for managing keys and signing:

$ helpeth --password 'Use --password-prompt instead for security' --keyfile UTC--2016-03-17T19-06-57.064Z--15f2f3e0f2d74ea7b185fc12f24cb4f402cc96d0 signMessage 'Hello World'
Input message: Hello World
Message hash (Keccak): 0x592fa743889fc7f92ac2a37bb1f5ba1daf2a5c84741ca0e0061d243a2e6707ba
The signature: 0x167760997a69e225c0668e6761cd20cac70f3a6ace29fe2d287c3003daf6972b10d158a47e8f064cf982a3defdf236247c41249dbfb0fb81f0d126c26a94971d01

As mentioned in this answer, if you have access to an RPC node or geth, the easiest way is to use the builtin eth_sign functionality.

There are various libraries for Javascript which let you do that manually if you are not afraid of writing some code. An example is shown by Niran.

If you have (or are happy using) the node.js environment, you can try out helpeth which is a command line tool for managing keys and signing:

$ helpeth --password 'Use --password-prompt instead for security' --keyfile UTC--2016-03-17T19-06-57.064Z--15f2f3e0f2d74ea7b185fc12f24cb4f402cc96d0 signMessage 'Hello World'
Input message: Hello World
Message hash (Keccak): 0x592fa743889fc7f92ac2a37bb1f5ba1daf2a5c84741ca0e0061d243a2e6707ba
The signature: 0x167760997a69e225c0668e6761cd20cac70f3a6ace29fe2d287c3003daf6972b10d158a47e8f064cf982a3defdf236247c41249dbfb0fb81f0d126c26a94971d01