www.volunteersouthamerica.net
if you're travelling on the cheap in central or south america and want to give something back, here's a great site listing volunteer opportunities that don't involve paying thousands of dollars to some middleman gringo organization offering "volunteer vacations".

Community Bike Programs: Worldwide Directory
taking over the world - one bike at a time!
Check out the rest of the ibike.org website too - it's awesome.

Maya Pedal - building sustainable bike-based machines
how cool is this!

crazyguyonabike.com
this is a great resource for anyone into cycle touring or reading about it - hundreds of personal journals, chocked full of information about touring realities all over the world, and tons of other info, all in a simple non-commercial format.

trailjournals.com
a similar service, but for long distance trail hikers.

bikerevolution.org
A free international bike registry, dedicated to stopping bike theft...everywhere.

warmshowers.org
another great resource: connecting bicycle tourists and those who generously make their homes available to those of us out on the road.

couchsurfing.com
a global community project, connecting travellers and couches around the world.

carbusters.org
an online magazine promoting a carfree world.