There are several implementations available that attempt to hinder an exploit by limiting the memory segments that code can execute in.
Solar Designer architected a kernel modification to x86 Linux to prevent execution in stack pages. Exploits will not be able to run their own code if the buffer resides on the stack (which most buffer overflows do) [16]. Sun also built an optionally enabled non-executable stack into the Sparc version of Solaris.
PaX is a x86 Linux kernel modification to mark all data pages non-executable, not just stack pages. PaX inhibits heap exploits in addition to stack overflows [14].
There are several overflow exploits that non-executable pages do not inhibit. By far the most common is the ``return into libc.'' Instead of executing custom exploit code, the attack directs the return pointer back into code can have malicious consequences depending on its arguements. The easiest example is to point the return address back at the system() library call and point the arguement at an instance of ``/bin/sh''.