Gnutella is one of those historic things that can potentially change completely the way copyrights are enforced. When it took off, it outdid Napster exponentially.

Nullsoft (Yes, the makers of WinAmp) published an Open-source beta release of Gnutella 0.1. I believe slashdot covered it, and the program and source was removed from the site within hours.

Why yank it so fast? Nullsoft is now fully owned by AOL, and extrememly recently before that, AOL merged with Time-Warner. TW owns a large record company, so it's not in their interests to release this type of program.

Luckily, there were so many people who already downloaded it, and the program wound up on FTP sites around the world. It's now completely out of the hands of Nullsoft, and has improved vastly, and is on every platform. Clones such as LimeWire and Bearshare are very popular now.

Gnutella did better than Napster and Scour, and continues to this day due to a simple concept; decentralization. Some clever programmer at Nullsoft realized that Napster and Scour had a simple flaw; shut down the main server and the network stops. If you could make a decentralized network of nodes that managed to not only download peer to peer but search themselves p2p and connect to others. Every client that signed on was able to locate other clients on the network, and share files and search across the network without a central authority.

Once that sort of network went up, it never went down. Open source developers augmented the code, allowing for NAT and firewalls, improved search speed, the ability to autoconnect to clients and interact with other gnutella distributions that were making their presence known. Today, it has copied many ideas from its competitor Kazaa, and implemented a two-level peer system, where some nodes can become Ultranodes, and forward traffic. This helps alleviate Gnutella's biggest problem of the time, scalability.

There are many popular gnutella applications that implement the protocol and can share amongst themselves. Many make use of the same Open Source libraries:
Acquisition for OS X

Doesn't a grassroots movement that grows exponentially give you a warm, tingly feeling?