Thetube.com has a journey planner, telling you the best way to get from one station to another. Trouble is, it only lists London Underground stations, so it's useless for telling me how to get to Dalston Kingsland tonight.
For that, you have to ask a French website: Subway Navigator, which allows you to choose mainline and Thameslink stations too. And not just London - it will enable to plan your journey in loads of cities worldwide.
In fact, after browsing this site for the last fifteen minutes, planning imaginary journeys across Toronto and Stockholm, I declare it to be the best damned website in the world!