General purpose sources:
Major portal dealing with Polish culture, with a focus on the arts, maintained by the Adam Mickiewicz Institute. In Polish, English, and Russian.
Internet version of largest Polish encyclopedia before the advent of Wikipedia.
Biographical portal maintained by Filmoteka Narodowa - Instytut Audiowizualny. Useful for historical biographies, not for contemporaries. Site can be a little slow.
Polish National Library.
Catalog of materials held at American libraries (and some national libraries outside the U.S.).
Additional source for birth/death dates and biographical details.
This publication is from 1996, so only useful for first three quarters of 20th century (and earlier), not for contemporaries.
Major reference source for music topics, including biographies of performers and composers.
Filmmaker and actor biographies and filmographies.
Images and other digitized content
Polish digital library collections.