www.szukajwarchiwach.pl is the official website of Polish State Archives. Over 2 million documents have been added recently and now the whole collection exceedes 28 million scans published online.
Besides vital records it contains scans of local censuses, passport applications, students registers, memoirs, cadastres and many many other resources useful in family research.
In one of my previous posts I explained how to use the database.
TIP:
try surname search as more and more indexed records are available