Deutsche Guggenheim (Monday Free)

Deutsche Guggenheim has free entry every Monday. Some of the exhibitions might not be available, but still worthy to visit this museum.

Location: Unter den Linden 13/15, 0117 Berlin
Official Website: http://www.deutsche-guggenheim.de/index_en.php
Opening Hours: Daily 10 a.m. to 8 p.m.
http://www.deutsche-guggenheim.de/vi_opening_hours_full.php

Max Plank Science Gallery

This is a pretty cool small free exhibition from the Max Plank Institute. Good spot to browse trough science studies and innovation, presented in sleek touchscreen pannels and cool infographics.
Definetely worthy to spend half an hour there.

Official site: http://www.max-planck-science-gallery.de/?lang=en
Opening Hours: Mo-Mi, Fr: 10 - 18; Do: 10 - 20;Sa-So: Closed
Address: Markgrafenstraße 37 10117 Berlin (see link below to Google maps)