Map My Emissions gives the greenhouse emissions for the route between any two addresses in the world (as long as the route is available on commercially available maps). You can choose between public transport, walk, cycle & car, and compare the outputs. You can also update the type of car to get more accurate results.
EcoPassenger gives the comparison of atmospheric emissions & energy consumption for train, aeroplane & car travel routes between two European cities. You can update your car specification, number of passengers, etc. to get more accurate results.
myclimate has calculators that will tell give the CO2 emissions of a flight (between any two airports in the world) or cruise, which could be used to compare with results from the above calculators.
Note that all calculators use different methods to calculate the emissions so only an approximate comparison between two different calculators is possible.
Shrink That Footprint gives the Carbon intensity per kilometre of different types of travel in general, CO2 emissions per kilometre from driving alone, overview of vehicle manufacturing emissions, and other useful information about travel footprints.
If driving, this US govt source lists the factors that affect fuel economy & in turn environment friendliness.