Frustrated by trying to coördinate different modes of transport to get from Switzerland to a conference in Poland four years ago, Jochen Mundinger had an idea for a search engine that would find the fastest, cheapest, or most ecological way to get from A to B. Mathematically, what's involved is a network optimization problem under a particular set of constraints--a perfect fit for Mundinger, who was trained as a network analyst and works at the Swiss Federal Institute of Technology in Lausanne. In 2007, he turned the idea into a company, RouteRank.

Type in, say, "Basel" and "Munich" in ­RouteRank and the system will find not only flights but also trains, public transport, and driving routes--and let you combine them. A click of your mouse lets you sort the results according to what you consider most important, whether that's price, travel time, or environmental considerations. RouteRank calculates the carbon dioxide emissions associated with each itinerary and lets you offset them by connecting you to Myclimate, a Swiss-based nonprofit foundation. Today RouteRank provides customized commercial service to Nokia, the conservation group WWF, and, most recently, the Swiss government. One surprising discovery, says Mundinger, is that the fastest way of getting somewhere isn't always the least green. Next steps include expanding beyond Europe.--Giselle Weiss