[pwpwp]

My favorite is the "weak" attribute. http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

Makes separate compilation for languages that compile to C much easier.

[DarkShikari]

My favorite would be "pure" -- it's perfect for saying "this function pointer, which calls an assembly function, doesn't muck with any of your data structures, so you don't have to reload every single pointer after it returns".

But unfortunately, since it was basically written only for libc purposes, it doesn't work on function pointers, so it's useless for this purpose.