This page is a bookmark of what resources are available.
Ethereumj - java implementation of the yellowpaper. https://github.com/ethereum/ethereumj
Ethereum Harmony - Ethereum network private peer based on EthereumJ with nice web interface
Ethereum Tester - python tester for Ethereum based applications, can be used to test private key and accounts, include functions of eth-account
eth-account - in python, Sign Ethereum transactions and messages with local private keys
eth-keys - python, ethereum keys handling library, donated
ethereumjs/ethereumjs-devp2p - nodejs, lower-level p2p connection and message exchange
ethereumjs/ethereumjs-tx - create transaction and sign with private key (this is the way in web3 document to sign a raw transaction from a private key)
ethereumjs/ethereumjs-util - utilities, including ecsign / ecrecover, generateAddress, isValidAddress, isValidChecksumAddress, isValidPrivate, isValidPublic, etc. : address handling, signing, hash, algorithms, etc.
https://github.com/ethereumjs/ethereumjs-block - encoding / decoding, validation of Ethereum block schema
ethereumjs/keythereum - javascript, generate/import/export Ethereum keys, can be used for verification of cold wallets, same key derivation functions etc. as geth. create random keys.
ethereumjs-wallet - supports key creation, conversion between formats. lightweight and work in browser
https://etherscan.io/verifySig