LGTM -- thanks @prantlf ! I'll look forward to the docs merged next. :)

Let's see. I'm going to have a look at the cpp-netlib documentation structure, but before "going wild" with the programmer's guide I wanted to try the two possibilities how to design the API - separate function templates or one class template. I tried to continue with the separate functions in the base64_util_function_templates branch and not being able to typedef the specialization is annoying me:

std::string result = base64::encode<base64::normal, char>("...");

std::back_insert_iterator<std::string> result_encoder(result);
base64::encode<base64::mime>("...", std::back_inserter(encoder), rest);
...
base64::encode_rest<base64::mime>(encoder, rest);

std::stringstream output;
output << base64::io::encode<base64::normal>("abc") <<
      base64::io::encode_rest<base64::normal, char>;

I'd like to try the static class approach and see what will annoy me there :-)