Craigslist and Kijiji - local classified ads, click on rideshare option. (Worldwide)
Couchsurfing - Just a great website, some cities have a rideshare group.(Worldwide)
Freewheelers - International Lift Share (UK)
http://www.freewheelers.co.uk/
liftpool - rides - lifts - The free ride sharing scheme for the United Kingdom and Europe.(UK)
http://www.liftpool.co.uk/
National carshare, Ride sharing throughout the UK(UK)
http://www.nationalcarshare.co.uk/
234car - car share, ride share, lift share, innovative personal transport solutions (UK)
Amigo Express - Quebec and Eastern Canada Rideshare. (CA)
HelloStop - $15 Weekly Rideshare between Quebec and Montreal (havent used myself, pls give feedback) (CA, Quebec)
E-Rideshare.com - National Rideshare (CA)
Allo Stop - Rideshare in Quebec and some of Ontario and the Maritimes. Charges a small membership fee but is reliable. (CA) Allo Stop - Quebec Rideshare
Facebook - I just discovered a Facebook Group for people who travel between NYC and Montreal. Really helpful! - (CA & US) People who travel between Montreal and NYC (Facebook)
http://www.234car.com/
Ridester (US)
www.Ridester.com
GishiGo (US)
http://www.gishigo.com/
Car Rental
Vagabondo Car Rental Comparison (IT, World)
http://en.vagabondo.net/xar_ext/car_rent.html
Bus
Autobuses Americanos - Bus Travel between US and Mexico (US/MX)
http://www.autobusesamericanos.com.mx
TicketBus (MX)
https://www.ticketbus.com.mx/ticketbus/inicio.jsp
Hitchhiking