LEGOizer

There are a few mosaic creation programs out there, but this is (so far) the only web based one. Drag an image onto the canvas, wait a moment, and you're presented with a parts list, instructions and the finished image. There aren't many settings, but it's a good starting point regardless. If you're into code (hello, me!) there's an interesting post on optimizing the app written by a third party. Open source is fun, kids!