Comparative Study of Electoral Systems (CSES)
http://www.umich.edu/~cses/

Comparative National Elections Project (CNEP)
http://www.cnep.ics.ul.pt/index1.asp

Elections around the world (Wikipedia)
http://electionworld.org/

Eurobarometer
http://ec.europa.eu/public_opinion/standard_en.htm

European Election Studies (ESS)
http://www.ees-homepage.net/

Extreme Right Electorates and Party Success (EREPS)
http://www.politik.uni-mainz.de/ereps/

PIREDEU (Providing an Infrastructure for Research on Electoral Democracy in the European Union)
http://www.piredeu.eu/

European Social Survey
http://www.europeansocialsurvey.org/

European Values Study
http://www.europeanvaluesstudy.eu/

International Social Survey Programme
http://www.issp.org/

World Values Surveys
http://www.worldvaluessurvey.com/

Database about parliamentary elections and political parties in Europe
http://www.parties-and-elections.de/