SQD (formerly known as Subsquid) is a custom blockchain indexing toolkit that allows developers to develop and deploy custom GraphQL APIs and ETLs for advanced blockchain use cases. It provides a dev-friendly way to build and customize indexing solutions for various blockchains, including EVM and Substrate chains. With support for over 100 networks, Subsquid offers fast indexing speeds and syncs, making it a valuable tool for developers in the Web3 space.