A privacy-focused P2P framework from the GNU project:

GAP (GNUnet Anonymity Protocol): query-based anonymous file sharing with plausible deniability.

R5N: a Kademlia-like DHT with censorship-resistance properties and query privacy.

GNS (GNU Name System): a decentralized name system with query privacy—an alternative to DNS that doesn't leak what names you're looking up.

Academic-grade cryptographic rigor; slower to gain mainstream adoption but influential in P2P privacy research.

A pioneer in censorship-resistant storage:

Content is encrypted and distributed across nodes; no node knows what it stores (plausible deniability).

Small-world routing for scalable lookup without a global DHT—routing adapts to the content topology.

Locutus (Freenet 2): a modern rewrite aiming for a general-purpose decentralized platform with CRDT-like contracts and WebAssembly components.

Important precedent for the anonymity dilemma.

Least-Authority File Store—a provider-independent encrypted storage system:

Files are erasure-coded, encrypted client-side, and distributed to storage servers.

Servers cannot read the data they store (privacy by construction).

Capability-based access: read-caps, write-caps, and verify-caps as unguessable cryptographic tokens. The cleanest practical implementation of the capability model.

Demonstrates that erasure coding + client-side encryption + capabilities can provide durable, private, trustless storage.

Continuous, real-time file synchronization between devices:

Block Exchange Protocol (BEP): devices exchange blocks of changed files using content-addressed block hashes for efficient delta sync.

Global Discovery and relay servers: assist NAT traversal and device discovery when direct connectivity fails.

Conflict resolution: last-writer-wins with conflict copies created for simultaneous edits. Simpler than CRDTs but effective for file sync.

Strong focus on usability and the edge-first pattern—no cloud dependency, everything runs on your devices.

Open source with a healthy community of volunteer relay operators (an altruism-based incentive model).

A minimal protocol for decentralized social networking:

Events: signed JSON objects published to relays. Each event has a kind, content, tags, and a signature from the author's identity key.

Relays: simple WebSocket servers that store and forward events. Anyone can run one; users choose which relays to use.

Keys as identity: NIP-01 defines secp256k1 key pairs. Your identity is your key, portable across any relay or client.

Replaceable events (NIP-16): latest-timestamp-wins semantics for profile updates, contact lists, etc.—effectively LWW-Register semantics.

Hybrid model: relays are servers, but users are sovereign (can switch relays, use multiple simultaneously, export their data).

Growing ecosystem: social clients, long-form content, marketplaces, messaging, and Zaps (Lightning micropayments).

Compliance: relay operators face moderation decisions similar to Matrix homeserver operators.

Each system emphasizes different parts of the P2P design space: GNUnet and Freenet prioritize privacy. Tahoe-LAFS showcases capabilities and erasure-coded storage. Syncthing demonstrates the edge-first pattern with mainstream usability. Nostr explores minimal P2P social with maximal user sovereignty. All face the security challenges and compliance dilemmas common to open networks.