Here is a list of very useful data archives and other websites with data on elections, parties, public opinion and voting behaviour:
Data archives
Danish Data Archive
ICPSR, Inter-University Consortium for Political and Social Research.
Mannheim Eurodata Archive, official statistics from European countries.
Norwegian Data Archive
UK Data Archive
Election studies
American National Election Studies (ANES)
British Election Study (BES)
CSES, the Comparative Study of Electoral Systems. Election studies from around the world, including comparative data on political institutions.
Information on elections and parties
Election world, details on elections around the world
European Commission, Public Opinion, details on Eurobarometer surveys and other datasets.
Lijphart Elections Archive
Parties & Election, a database about parliamentary elections,
parties and political leaders in Europe.