OpenBCI is an eight channel EEG system (on their source codepage, there are instructions on how to increase the number of channels!), that runs on top of Arduino (although the use of other microcontrollers are possible). From the project website, they successfully finished a kickstarter campaign and are going to start selling this spring. But being a open source project, one can already download the build plans and software code and start making EEG measurements.
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.