activehistory.co.uk - A premium subscription service from the UK that is now being offered for free. Lots of different types of resources including a large number of online history simulations.
Google Arts and Culture - Always free, always awesome! Virtual tours and collections of museums, historical sites and events. A great way to take a virtual field trip.
Virtual Museum of Canada - Again a free service but worth taking a look if you aren't familiar with it. Virtual exhibits and a teacher's resource centre.
Open Culture - One of the original free educational sites out there. Its an eclectic collection of resources but a lot of gems of digital content and links to other resources in there.
Please feel free to leave a comment and share any of your own go to sites and/or resources.