The Nerpa test network, often called Nerpanet, was designed with tiny sector sizes. This network was for application developers to test the very basic functionality of their applications before moving over to the calibration test network. It was deprecated on 2021-08-16.
The Spacenet test network was created for the Interplanetary Consensus (IPC) project. This network was deprecated on 2023-09-11.
The Wallaby test network, often just called Wallaby, was designed for internal Filecoin developers to test new features before rolling them out to the Hyperspace testnet, and then onto Mainnet. Wallaby was designed to be reset every week. It was deprecated on 2023-02-07.