Blockstack is described as 'new decentralized internet where users own their data and apps run locally. Take back control over your digital life. A browser portal is all you need to get started' and is an website in the bitcoin & cryptocurrency category. There are seven alternatives to Blockstack, not only websites but also apps for a variety of platforms, including Mac, Windows, Linux and Self-Hosted solutions. The best alternative is IPFS, which is both free and Open Source. Other great sites and apps similar to Blockstack are Freenet, Hypercore Protocol, Solid and Decentralized Internet.
Blockstack is mainly a Anonymizing Network but alternatives to it may also be File Sync Tools. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Blockstack.
IPFS is a global, versioned, peer-to-peer filesystem. It combines ideas from Git, BitTorrent, Kademlia, SFS, and the web. It is like a single bittorrent swarm, exchanging git objects. IPFS provides an interface as simple as the HTTP web, but with permanence built in.