Is there a tool that generates P/Invoke signatures for arbitrary unmanaged DLL?

I use PInvoke Interop Assistant for unmanaged DLLs by using the third tab in the UI, marked "SigImp Translate Snippet". Simply copy-and-paste your header into the "Native Code Snippet" window and press Generate (or turn on Auto Generate). As an illustration here's some code from a question of mine. Note that for some reason errors don't appear in the Error panel but as comments at the top of the generated code.

As several people have already said, the generated code should be used as a guide - you may well have to make changes to get exactly what you want.

Google quickly found http://www.pinvoker.com/ (Compatiblity listed as VS2005, 2008, and 2010; it doesn't seem to have been updated to work with newer versions)

Microsoft's C++/CLI compiler can also do this, if you use /clr:safe and #include the header file, it will generate p/invoke code which you can extract with e.g. ILSpy (free) or Red Gate Reflector (used to be free).