Mówić po polsku: This site has a wide range of free Polish language classes. It has modules on everything from pronunciation to grammar.
Wikibooks offers Polish language learners a free online coursebook.
Live Lingua: There are several free courses in Polish on Live Lingua, a database of Peace Corps and FSI coursebooks and audio materials.
Elementary Polish is a free basic Polish course created by a faculty member at the University of Chicago.
Polish Grammar features free Polish grammar lessons as well as flashcards and information about Polish culture.
I Will Teach You Polish: Pawel teaches you Polish through texts with audio. There are lessons on specific topics like going to the movies, but he also breaks down the lyrics to different Polish songs.
Goethe Verlag has 100 free lessons for Polish language learners. They cover everything from numbers to preparing for a trip and running errands.
BBC Languages: Looking for a fantastic introduction to the Polish language with video and tons of accompanying material? The BBC’s Polish course has it all.