Since a few months ago, my kids have become big fans of Random House's Magic Tree House books, written by Mary Pope Osborne. In these books, a couple of kids by the name of Jack and Annie accidentally find some day a magic tree house that transports them in time to the age of the dinosaurs, pirates, the Middle Ages, etc. In each adventure, the kids have a mission to carry. After a while, I found myself searching the Internet to find website with pictures on Pompeii, the Titanic or whichever other topic related to the books we were reading. Finally, I decided to bundle them together in this page, instead of trying to keep track of them in my browser's bookmarks collection. As you can imagine, I chose to pick those websites that have more pictures, flashy graphics, etc.

Collection of images of the old Roman buried site organized by topic: Villa of the Mysteries, House of the Vetii, homes, baths, mosaics... It also includes links to other resources.
Excellent collection of pictures of the Roman site, including a massive list of thumbnail pictures. More comprehensive than the link above, it also has pictures of the amphiteather, the gladiatorial barracks, individual houses, streets, temples, tombs and even a lupanar.
Companion website to PBS's The Vikings, a two-hour show on the famous Norsemen. It's a pity that all their links to video resources are currently brokne, but there are still plenty of pictures, resources and texts to enjoy.
Companion website to the Smithsonian Museum's itinerant exhibition on the Vikings. Heavy on Shockwave plugins, but quite interesting.
This site requires previous registration, but it's free anyways and it contains a lot of great resources on the history of Ancient Greece.
Pictures of Athens' most important monuments: the Acropolis, Philoppapos monument, the Agora, the Olympieion, etc.
Pictures of the exterior, the interior, information about the ship, the people who were aboard, etc.
Nice collection of pictures of the boat and some of the people who were aboard. The site also has a couple of links to short videos that are quite interesting.