The Onion Router

https://www.torproject.org/

A low-latency source-anonymising overlay network for TCP (and over TCP). The tor client acts as a local SOCKS4A/SOCKS4/SOCKS5 proxy running on port 9050. A command line tool 'tor-resolve' is also included. If an application doesn't support SOCKS4A, you can use freecap (Windows) or tsocks (Linux) to route all traffic through tor.

AHBL provides a tor DNSbl:

To see if 10.20.30.40 is a tor exit node or not, query the A record of 40.30.20.10.tor.ahbl.org

 dig 40.30.20.10.tor.ahbl.org

Harvard has a nice list of onion routers with WhoIs links at (updated twice an hour):

http://serifos.eecs.harvard.edu/cgi-bin/exit.pl (BrokenLink 2006-11-08)

And the following site lists onion router IPs (updated at least daily):

http://torstat.xenobite.eu/

The number of onion routers worldwide has grown from about 40 in Oct 2004 to over 800 in Oct 2006. The greatest numbers are in Germany and the UnitedStates.

Any reasons for the increase? Is the trend continuing at this rate?

(I see you're using TOR!) The growth rate in the past 24 months is plotted at http://www.noreply.org/tor-running-routers/totalLong.html. You can see the rate is uneven but climbing. The TOR server software was first released in October 2003, and version 0.1 came out in June 2005. The most recent version as of this writing is 0.1.2.1-alpha in August 2006. Considering its low version number, this product seems rather experimental. Yet it's obviously catching on.

TOR can lead to BondageAndDisciplineNetworking