The political system
A good guide to the workings of the Bundestag, including how legislation is passed, ‘The Bundestag at a glance’ can be found here.
Information about the upper house, the Bundesrat, can be found here.
The Federal Agency for Civic Education has information about policy issues, some of which is in English.
Background information on the main parties in Germany can be found on the Deutsche Welle site here.
The German constitution
Wahlrecht complies all national polls under the heading ‘Sonntagsfrage Bundestagswahl’, and also has pages for polls for state elections.
Information about politicians
Abgeordneten watch has information about how individual MPs and parties voted in all votes in the Bundestag and state parliaments, as well as information about second jobs.