This is the most powerful and moving place.  A visit to the Holocaust Museum, the Genocide Museum and the Jewish Museum on the same afternoon is not exactly a day to lift the spirits, but it gives an idea of some of the horrors to have occurred in Lithuania over the course of the lifetime.  Along with the Genocide museum, this place is the must-visit 'attraction' if you have any interest in history at all. (The Jewish museum is more a tribute area, giving insights into the personal thoughts, and though moving is the least informative).

The information given around the museum is very well written, and manages to describe the horrors very powerfully, but unbiased way given that it seems to have been written by people who knew only too closely the enormity of what went on, and also that many Lithuanians were complicit in many of the atrocities.  As a result, and combined with the photographs and other exhibits, the effect is incredibly powerful.  It must make even more uncomfortable reading for Lithuanians than it does for visitors.

Totally, totally recommended, if you don't mind being upset.