Today, Sarah and I visited the National September 11 Memorial, which sits on Ground Zero.
Visiting the memorial requires a visitor pass. The passes are free, but have to be reserved ahead of time on their web site. (I reserved our passes in late April, and there were already no slots available on Friday, Saturday, or Sunday.) There was about a 15 minute wait to enter the memorial, which included passing through metal detectors.
The memorial consists of a plaza full of trees, and two recessed pools in the footprints of the twin towers. Water flows down the sides of the pools, and the names of 9/11 victims line the rims of the pools.