Allowing third party users to upload files into your AWS S3 fs

I have a pretty tough problem here: I need to allow the users of my site to upload very large files to their accounts and I want to store these files on a AWS S3 filesystem. I can't just write a web service to receive these and save them in the S3 fs, because all kinds of things can go wrong during the upload and I need a sophisticated uploader client. The kind of client that Amazon provides to upload files into S3, but of course I can't give my users direct access to that. I'd seriously appreciate any ideas for this!

Thank you