[nitrogen]

The C99 standard itself and corresponding Wikipedia pages are quite useful. The gcc manual is also a handy reference for determining whether a particular gcc feature is standard compliant.

http://www.google.com/search?&q=filetype%3Apdf+iso+9899

http://en.wikipedia.org/wiki/C99

[reirob]

Funny that only 2 commercial compilers are fully supporting C99 standard: IBM Rational logiscope >= 6.4 and Sun Studio.