A more recent post can be found here.
Doors Open Toronto is a great concept and a great event!
Doors Open Mississauga also warrants celebration; it takes place on September 28, 2013.
Doors Open Ontario offers information about the history of the Doors Open concept and provides a link to Doors Open events in other countries. At the latter link, you can find some interesting live links and some that are not currently live.
The Doors Open Ontario link notes that the first Doors Open Day (La Journée Portes Ouvertes) took place in France in 1984.