Emotiv EPOC+ Research Edition can be used on Windows, Linux and Mac.
It has 14 electrodes (+2 reference electrodes) and is used in several publicized studies.
You can access raw data via their software "Testbench" and output it as CSV or use third party software like OpenVibe, BCI2000 and EEGLab.
But the little drawbacks are:
It is a wireless device, so you really should put the usb dongle for wireless signal transmission with a long usb cable directly on the device, to overcome signal distortion
The reference electrode processing is done within the device
It seems to be more intended for consumer use than for medical use
EDIT about realtime usage of EPOC+ and OpenVibe Software:
There are studies and projects using the EPOC+ in realtime with OpenVibe. OpenVibe has a built in driver for the EPOC+ Research Edition
And in this project two devices can be used, the EPOC+ and the the Enobio, which seems to be an intersting device, too. But I do not have enough knowledge about the Enobio to recommend it. But it might be even better as the EPOC+, as they have models with more electrodes. This is the realtime application for the two devices:
By the way, OpenVibe features drivers for quite some EEGs, but it can only be used on Windows and Linux. On Mac you would have to compile it yourself.