btctxstore simple library to store/retrieve information in bitcoin transactions using OP_RETURN
pybitcointools Python library for Bitcoin signatures and transactions from Vitalik Buterin. Project discontinued.
pycoin Python library for Bitcoin keys, signatures, transactions. Includes full VM implementation and tools for manipulating keys (ku) and transactions (tx).
bitcoin_tools Python library for building and analyzing transactions and scripts (both standard and custom). Comes along with a UTXO set analysis tool. Includes several examples and exhaustive documentation.