I've probably linked to Karl's Great Circle Mapper before; it's not a serious flight planning application, but it's a fun visualization tool for long flights, or for working out your frequent flier mileage. The site also has regular Featured Maps associated with news events, such as the intended route of a crashed aircraft, or the routes considered in planning for humanitarian missions to Haïti after the earthquake.
It's kind of fun using the chart to plot routes around the globe even if no one is ever going to fly them, and Karl occasionally issues little challenges. One was to find the longest domestic flight, the longest flight between two points in the same country. Try it yourself before you look at the suggestions some people came up with. In the end it doesn't matter whether you count territories as being part of the possessing country, or you require only politically equal regions of the country: the same country came out on top. But longest routes found are not in countries you might guess. Oh and zigzagging across the country through different airplane hubs doesn't count. It's the longest Great Circle route between two points in the same country.
I thought Canada had some pretty long potential routes, but I found it more fun to plot a circle that went all the way around Canada. It's not a flight anyone would ever do. I haven't even done any of the legs. The 2010 Olympic torch 'relay' probably came close.
The Google CEO cited Great Circle Mapper as his favourite website.