The latest trick those techies in Silicon Valley have brought to Google Maps? Transport directions.
From any Google Maps page, click on the 'Get directions' tab, then enter your two points and click on the 'By public transport' button. The map will then compute the best way to get there by Tube and / or bus, and suggest three optional routes; hovering the mouse over each one overlays the route onto the map, so you can pick or choose the best.
There's an impressive attention to detail on the page. Tube lines in the 'Suggested routes' pane are colour-coded, and in the more detailed route description, the number of stops and the approximate travel time are included. Add in the usual Google Maps tools, such as the ease of dragging and dropping to amend your journey points, and it adds up to an excellent package. Android users have it even better: a mobile app currently in beta, Transit Navigation, brings many of the same features to your smartphone.
While Google Maps is a welcome addition to existing transport direction sites, such as BusMapper and Transport for London's own Journey Planner, it's not without faults. For example, it doesn't appear to factor in engineering work: ask it to get from High Street Kensington to Bayswater and it'll blithely suggest you take the Circle or District line, oblivious to the fact that the Tube is currently closed between those stations. Also, as pointed out in the comments, it lacks data for suburban trains, including London Overground.
Those problems aside, it's an impressive entry in the increasingly innovative digital mapping market.
Hat tip: @movanet