PlanetFiles is not as centralized as, say, Napster. The central server only stores information about the users who are logged into the system at a given type, not a list of the files they share. People who are logged in can send queries to these users, or request lists from them, but all of this is done through direct user-to-user connections. (The server sends me a list of 10 IP addresses, I contact each of them individually, the server sends another list of 10 addresses etc.)
The advantage of this method is that it doesn't take much bandwidth, and that it would be hard to do any content-based filtering on the server side ("You're not allowed to share Metallicrap MP3s!"). This should also give PF a better legal standing than Napster et al. The disadvantage is that it's pretty slow to query a large number of users, so the number of users has to be narrowed first.
The server does store information about the content type the user is hosting (images, text, music) and whether this content has been classified as "freeware", "public domain", "shareware", "owned by user". Nothing else.
This concept is pretty interesting, but it will probably not scale to a large amount of users without further limitations. One idea I suggested is to allow users to search only for files in one or several particular chat channels. If you'd be willing to wait, you could still search hundreds or thousands of users.
PlanetFiles is not only interesting because of this concept, it's also interesting because of the implementation, which is the main reason I'm featuring it. It's a good, clean closed-source Windows program, the way I like them to be. No ads, no Internet Exploder integration, no stupid widgets. There are tabs for easy navigation and advanced menus for advanced users.
I think similar simplicity was part of the reason for Napster's success. (Yes, Napster does integrate IE, but it recognizes whether it is installed and if it isn't, doesn't display the IE integration parts.)
Perhaps most important, PlanetFiles is fast -- thanks to the simple, but aesthetically pleasing UI and a lot of clean programming underneath, it doesn't take long to generate lists even for hundreds of files, and they are displayed at high speed as well. Too bad it doesn't have ratings (allow users to judge the quality of files).
PF has a lot of decent search options (like automatically searching for similar files or files in the same path) and Napster-style hotlisting. Unlike Napster, it allows users to restrict both the total number of uploads and the number of uploads per user. The program is also pretty stable.
Two downsides: It uses a lot of memory (12 MB right now) -- that, and the network is still relatively small. A little annoying is the necessity to classify files by public domain, shareware etc. -- people will simply lie about that, and "We asked them not to do it" has never helped much in court. The standard notice in the license agreement should be enough. (Even the fact that this is considered necessary worries me, like you had to sign a license agreement "I will not make death threats" when you buy a telephone..)
Like Filetopia, which is in Spain, PlanetFiles is hosted in Europe (France). That's pretty good, since European countries are not as unified as American states yet, and a decision to shut down a server in France would not mean that a server in Spain would be shut down as well. "The more, the merrier!" Perhaps another file sharing service for Germany, one for Sweden, one for .. you get the idea. And if the server software is released as well, you'll have servers popping up everywhere (like OpenNap).
The question is, where should users concentrate? In my list of recommendations, PlanetFiles has climbed up the ladder pretty quickly. Give it a try.