We're big fans of Aussie-developed travel search engine rome2Rio because of its ability to calculate routes that include trains, buses and cars as well as planes. The site now has another useful trick up its sleeve: it can calculate the carbon impact of your journey.
Using data from Offset Options, the site now calculates your total impact across multiple modes of transport, and gives you the option of contributing to a specific project to help offset that. That's simpler than trying to purchase individual offsets for each leg, especially in contexts (like renting cars) where that isn't an option. It's also a handy way of calculating the rough carbon impact of any journey.