As of version 4.1.0, BitTorrent now includes "trackerless" operation.
While it is called trackerless, in practice it makes every client a
lightweight tracker.  A clever protocol allows clients to efficiently
store and retrieve ip-address/port information for torrents.

When generating a torrent, you can choose to utilize the trackerless
system or a traditional dedicated tracker.  A dedicated tracker allows
you to collect statistics about downloads and gives you a measure of
control over the reliability of downloads.  The trackerless system
makes no guarantees to reliability but requires no resources of the
publisher.  The trackerless system is not consulted when downloading a
traditionally tracked torrent.

The trackerless protocol runs over UDP.  If you are forwarding a TCP
port on your firewall to enhance BitTorrent operation you should also
forward the same UDP port.  Many NAT devices, including wireless
access points and DSL routers, automatically forward UDP ports.  Even
if your particular arrangement doesn't allow for reliable service to
others in the trackerless protocol, your client should still be able
use the system to find peers.

Each client in the trackerless system maintains a routing table of
some of the other clients in the system.  No client has a complete
directory of every other client in the system, it is distributed among
all clients.  To find another client, an iterative protocol is used to
find closer and closer clients, until the desired one is found.
Before this process can be started, the routing table must contain at
least one valid entry.  Clients acquire the contact information of
other clients during the course of normal use, but a newly installed
client has no contacts and no way of entering the network.

There are two ways a newly installed client obtains contacts for the
trackerless system.  The first is through tracked torrents.  When
connected to another BitTorrent client that supports the trackerless
system, the peers will exchange contact information for the
trackerless system.  The other way to get bootstrapped into the
trackerless system is by using contact information stored in
trackerless torrent files.  When you create a trackerless torrent,
contact information from your routing table is automatically included
to help newly installed clients with no contacts.  An alternative is
to include the contact information of known stable clients or special
"routing" clients.  A routing client doesn't provide torrent tracking
service, it only provides contact information.  When specifying the
contact information for a trackerless torrent, separate the IP address
and port with a colon and separate contacts with a comma
i.e. "router.bittorrent.com:6881, 127.0.0.1:8080"