General Information:
EU-Community: http://eu-community.daad.de/index.php?id=73
EURES-Portal: https://ec.europa.eu/eures/main.jsp?acro=lw&lang=de&catId=490&parentId=0

Visa: 
Visa types and requirements: https://www.msz.gov.pl/en/travel_to_poland/visa/visa

Entry and residence requirements:
Ministry of Foreign Affairs: msz.gov.pl/...

Social insurance provisions:
ZUS - Social Insurance Institution: zus.pl/...
Ministry of Labour and Social Policy: mpips.gov.pl/en/

Accommodation:
Just landed - Housing & Rentals: justlanded.com/...
Bed & Breakfast: https://www.bedandbreakfast.com/poland.html

Country information:
Wikipedia: wikipedia.org/...
Kwintessential - Doing Business in Poland: kwintessential.co.uk/...
Culture Crossing: culturecrossing.net/...

Further information:
Poland Travel: poland.travel/...