Due to a trip plan to an European city where I honestly didn’t knew if the subway was an option on transportation. I came up with a short but useful list of maps (resources) for subways across the world. Hope you find it as useful as I did (will). Some of the maps need a severe update but they’re still good enough for the typical trip planning.

UrbanRail.net - Hundreds of subway maps. You can also find very useful information on historical facts, ticket prices and some links to the official websites whenever they’re available.

Subway Maps (Amadeus) - Not really a website. Just one of the tools for travellers made available by Amadeus.net.

Subways - A very geek-style list of map resources.

PSP Subway Maps - Post by Endgadget (2005) with links for PSP usable maps. Still, I think you should keep your PSP on you bag when travelling.

Happy planning!