Freddy Mang has an online nimstring calculator
but unfortunately it doesn’t always work correctly!

I wrote a corrected and improved (more efficient) version
that is available at the following URLs.

http://paul.rutgers.edu/~rhoads/nimstr.zip
http://paul.rutgers.edu/~rhoads/nimstr.tar.gz

Both files contain the same stuff; the first one is a
zip file while the second is a gzipped tar file.