OpenBCI is a complete open source EEG system that can be built either on top of an Arduino (8-bit system), or on top of chipKIT (32-bit system), which gives the system more local memory and allows for faster speeds.
The openEEG project aims at describing and putting manuals for building a two channel EEG system for about U$200. More on instructions on how to build one, can be found here.
OpenHardwareExG: is a project that provides both open source hardware and software for the measurement and analysis of different types of biosignals From the project page: About the OpenHardwareExG project Project goals The main goal of the project is to build a device that allows the creation of electrophysiologic signal processing applications.