Strictly speaking, the C standard is not available for free. You need to purchase an electronic copy from http://www.ansi.org/ or http://www.iso.org.
However, you can download a free draft version from the Open Standards Organization's website - this is as good as or better than the official standard itself. It incorporates information from the ISO C99 standard plus three Technical Corrigenda, all of which are official ISO documents.
More info here: http://c-faq.com/ansi/avail.html