If you want to manage one of your MetaMask accounts from another Ethereum client, MetaMask tries to make this as easy as possible.
First of all, your 12-word seed phrase is compatible with most other Ethereum wallets, and you should be able to import it into another wallet fairly easily.
Secondly, if you only want to export a single account to another wallet, you can click the "Export Private Key" option on the Account Detail view. Currently, that's the key icon:
It should also be easy to import a private key into most other wallets. Let us know if you have any issues!