Biblioteka Narodowa

Even if you don't read Polish, the National Library is worth a browse for its historical volumes and rich architecture. The largest library in Poland, the National Library dates back to the beginning of the 18th century. Though it has lost much of its collection through subsequent invasions of Warsaw through the years (including more than 400,000 volumes being 'relocated' to St. Petersburg following the suppression of the Kosciuszko Uprising in 1794 and 1795 and then heavy destruction during the Warsaw Uprising), the present collection houses over 5 million volumes, including some of the oldest documents written in the Polish language.

Interesting, but I like the University library better :)