Tectonicus generates highly details maps of Minecraft worlds, and lets you explore them via google maps, with panning and zooming. You can then upload the generated map to the web and let other people looks around your map.
Tectonicus is completely free, but if you like it then please consider donating to charity in it’s name. There’s a donations page.
People who donate get a ‘supporter’ icon next to their player on all maps.
For more information on running Tectonicus, see the page on the minecraft wiki.
You can also join the discussion thread on the minecraft forum.