Off the top of my head, C99 features that I've used in the past couple months:

  - declarations mixed with statements
  - declarations in `for` constructs
  - hexadecimal floating-point literals and conversion specifiers
  - the strtoxx( ) conversion routines
  - the restrict qualifier
  - the expanded math library
  - designated initializers
  - extended integer types
  - compound literals
  - // comments
  - VLAs
  - complex support

Note that some of these features are still not present in C++11.

C11 features that I have used in the past few months:

  - CMPLX macros
  - anonymous structures and unions
  - <stdalign.h>