Or rather 6 images of million man marches* (millionyyat) super-imposed on each other.
This was created with a Gimp plug-in that I wrote. This has a limitation of superimposing up to 100 photos.
Here are other samples created by the plug-in.
Google+, created from a gif animation by Micaël Reynaud.
All the results of the Google image search to the keyword 'Hosni Mubarak'.
Same search as above but for pyramids.
Google image search for keyword 'street'.
Google image search for keyword 'bike'.
* It's difficult to have an accurate estimate of the true number of people who visited the square on a given day and a single photo may display a fraction of the people who showed up on that day.