Useful for all learners:
Bab.la – A multilingual dictionary, shows translations in a wider context of quotes from real texts.
Forvo.com – An ever growing guide to pronunciation created by the international community. All audio files were recorded by native speakers.
Useful for learners on level B1 and above:
certyfikatpolski.pl – The official source of information on national certificate exams, where you can find out about requirements, prices, places and times of upcoming exams.
Biblioteka internetowa Wolne Lektury – A collection of royalty free e-books and audio books in Polish, in many popular digital formats. Among others, you can find classics of Polish literature and some older translations of foreign books.
Grzegorz Jagodziński – gramatyka polska – Probably the most complete description of Polish grammar available on the internet in Polish and in English. Recommended for those really interested in how the language works, especially with a background in linguistic education.
Słownik języka polskiego PWN – The dictionary by Polish Scientific Publishers PWN, where Polish people go, when we have doubts about our own language 🙂
Narodowy Korpus Języka Polskiego – The National Corpus of Polish, an advanced tool you can use e.g. for checking on excerpts from real texts if a word is used in a given context.