The current standard for Programming Language C is ISO/IEC 9899:2011, published 8 December 2011, and known as C11. However, this has to be purchased from ANSI, ISO, or the BSI. Here we present the available draft, and thus free, versions of the standard.
The earliest standard was ANSI X3.159-1989, released in 1989, and referred to as ANSI C or C89. Before this the unofficial standard for C was Brian Kernighan and Dennis Ritchie's first edition of The C Programming Language, which was published in 1978. This original version is normally referred to as K&R C.
Available C Standard Drafts and Related Reports
The following are the the documents that are available for the C89/90, C99 and C11 standards:
The older C89 standard might be of historical interest. As a text file, the satandard is ansi-c89w.txt or ansi-c89u.txt (given here with Windows and Unix line ends). There is a more readable HTML version at port70.net/~nsz.
Other Related Standards and Reports
The following are a selection of interesting documents from WG14 and elsewhere.
Safe Library functions: The draft of this technical report, written in 2005, is N1146 (original from WG14). A rationale is in document N1147 (original from WG14); and N1135 (original from WG14) is also relevant to this issue.
Differences Between C89 to C9X by Thomas Wolf
Incompatibilities Between ISO C and ISO C++ by David Tribble
The clc-wiki C Standard is a good explanation of the various C standards, and has a more comprehensive list of sources than is given here.