Links to things that have been useful to me in my neurofeedback endeavors. Some of these I found through Googling, but many where pointed out to me by people on the OpenEEG list.

TI datasheet showing basic EEG amplifier with driven right leg (DRL) circuit (Figure 8)
Articles by Alexander C. MettingVanRijn et al. The files to look at are artikel1.htm-artikel8.htm, like this one (artike17).
Original Brainmaster schematics
One guy's homemade stuff
Self powered optoisolated RS232 interface design by M Asim Khan

Efficacy of neurfeedback of ADHD by Monastra et al. Good paper. Has protocol specifics for ADD.
Paper on electrode noise by E. Huigen et al. Very good paper.
Article in Wired archive explaining some of the basics
Some stuff on brainwaves by a company that sells stuff (The Transparent Corp)
Just a basic paper on Fundamentals of EEG measurement

Getting started with neurofeedback by John Demos. If you want to do neurofeedback training yourself, this is the book to get. It has details on how measurements are taken. The other books I've read give overviews and the history of neurofeedback, but not the specific details of how its done.

OpenEEG
Joerg Hansmann's site One of the designers of ModularEEG. Joerg is very active on the OpenEEG list. His answers to the many technical questions I posted to the list gave he a huge jumpstart into EEG measurements.
BioSemi Corporation which has a FAQ page
Some guy's stuff on brainwaves He explains the different wave frequencies and stuff.