Peer-to-peer filesharing

Introduction

eMule

Where can I get a reliable list of servers?

http://www.gruk.org/server.met.gz (still safe after the RazorBack servers were seized in Feb 2006?)
http://www.jd2k.com/server.met
http://www.linuxpourtous.com/server.m et
http://corpo.free.fr/server.met

Can I post a question as a comment on a file I'm downloading?

As of eMule 0.44d, this is currently only available if you downloaded at least one chunck. This feature has been requested.

Can I play an incomplete file?

Run the file by VLC, and see how it goes.

BitTorrent

Prior to version 3.2, BitTorrent by default uses ports in the range of 6881-6889. As of 3.2 and later, the range has been extended to 6881-6999. (These are all TCP ports, BitTorrent does not use UDP.)

BitTorrent will usually work fine in a NAT (network address translation) environment, since it can function with only outbound connections. However, you will get better speeds if you can accept incoming connections as well.

Some alternative clients for Windows:

BitTornado
Burst!
ABC [ Yet Another Bittorrent Client ]
BitTorrent EXPERIMENTAL download client
PTC a frontend to the original BitTorrent engine

Adding BT capability to a shared web server

The goal is to have both a tracker and a seeder on a shared server, as an alternative to HTTP or FTP downloads:

It seems like no single tool can act as both a tracker and a seeder, so you'll need to install those two separately, eg. PhpBtTracker as the tracker, and WebSeed as the http-seeding tool; Note that your users will have to run an http-seeding-capable BitTorrent client, such as BitTornado or ABC.)
As an alternative to WebSeed, TorrentFlux is a semi-BT client in PHP, but it requires Linux BitTornado (0.3.7 or Higher) or BitTorrent Client(3.3 - 3.4.2 +)

Webseed

http://bittorrent.radlight.com/webseeding.htm

Torrent Trader Lite

http://www.torrenttrader.com
No access control to limit who can upload a torrent or use the server as tracker?

TorrentFlux

"TorrentFlux is a FREE PHP based Torrent client that runs on a web server."
Requires BitTorrent 3.3 to 3.4.2 + or BitTornado 0.3.7?

PHPBTTracker

http://blog.monstuff.com/archives/000200.html
Requires MySQL

BlogTorrent

BlogTorrent is only a tracker; requires a separate host to act as seeder (?)
To upload a file, BlogT will download a modified version of BitTorrent, and install it silently in "c:\Program Files\BlogTorrent\
Once the BT client is installed, it will be triggered every time you click on "Upload a file", will generated a .torrent, and upload both files to the web server
"Download error : rejected by tracker - This torrent is not authorized on this tracker"

PhpBTTrackAdmin

http://phpbttrackadmin.sourceforge.net/
Under dev

Torrential

"Torrential is a PHP BitTorrent tracker based on TorrentBits. It features integration with phpBB3 "Olympus" as well as many other features."

ByteMonsoon

"Based on the original bytemonsoon tracker written by dfx, this project is to carry on the bytemonsoon legacy, making it the fastest and most configurable bittorrent tracker based on php and MySQL."

PHPBTTracker+

"PHPBTTracker+ is a bittorrent tracker; it is an extension of the original PHPBTTracker authored by DeHackEd."
ie., requires a separate BT client to act as seeder