Do you mean a dictionary where you can find the word you need, without knowing how to spell it? I don't know if there are any tools like this now, but hopefully someone else does. I'm sure many dictionary users find this issue frustrating.
This is probably a terrible suggestion (because of the still primitive nature of the tool), but have you tried speaking into google? Use voice recognition to type (beta) - Google Chrome Help They have "voice recognition" which might be a way to develop this sort of tool.
How do you imagine this tool to work? It seems like a clever idea, can you explain it a bit?