http://climexp.knmi.nl/ - A very large source of information providing numerous observations and allowing the user to make correlations between different fields or climate indices.
http://www.ifremer.fr/lpo/cours/elNiņo/index.html - A description of the El Niņo phenomenon in French.
http://www.elNiņo.noaa.gov/index.html - NOAA El Niņo page with forecasts, figures, 3-D visualisation, etc.
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter14/chapter14_03.htm - The chapter 14 of this online textbook is devoted to oceanic equatorial processes and in particular to El Niņo.
http://www.ldeo.columbia.edu/res/pi/NAO/ - A general site about the NAO, including useful links and publications (unfortunately not updated recently).