I'm getting tired of having to look these up every time I forget them, so here they are for my (and your) reference. [Source: Unabashed Librarian, #78:9-10, 1991.]

Seven Wonders of the Ancient World
This is the famous list compiled by the ancient Greeks and is what most people think of as simply the "Seven Wonders of the World". Of these, only the Pyramids still exist although the ruins of the Lighthouse have been found in Alexandria's harbor, and there are plans to rebuild the Colossus of Rhodes. The one problem with this list is that it is limited to the Mediterraian world that the Greeks would have been familiar with.

1. Pyramids of Egypt
2. Colossus of Rhodes
3. Hanging Gardens of Babylon
4. Mauseoleum at Halicanassus
5. Pharos Lighthouse of Alexandria
6. Statue of Zeus at Olympia
7. Temple of Artemis at Ephesus

Seven Wonders of the Natural World

1. Mount Everest
2. Victoria Falls
3. Grand Canyon
4. Great Barrier Reef
5. Prehistoric Caves in France and Spain
6. Paricutin Volcano, Mexico
7. Harbor at Rio de Janeiro

Seven Wonders of the Medieval World
There are a couple of odd choices here. Stonehenge is old enough to be classified with the Ancient Wonders, although this may not have been known when the list was compiled. And the Colosseum dates more from late antiquity, but I guess they wanted to honor it somewhere in a list of wonders.

1. Colosseum of Rome
2. Catacombs of Alexandria
3. Great Wall of China
4. Stonehenge
5. Leaning Tower of Pisa
6. Porcelain Tower of Nanking
7. Hagia Sophia in Istanbul (sometimes called the Church of Saint Sophia)

Seven Wonders of the Modern World
This was compiled early in the twentieth century, and many things have been built since then that might bump some of these from the top seven. Many organizations have created their own more recent lists, but each one is different, and none have been universally accepted to replace this one.

1. Eiffel Tower
2. Empire State Building
3. Suez Canal
4. Golden Gate Bridge
5. Dneproges Dam on the Dnepr River in Russia
6. Atomic Energy Research Establishment at Harwell, England
7. Alaska (Alcan) Highway

Ultimate Ten Man-Made Wonders of the World
Wonders that still exist from all times and places.

1. Great Wall of China
2. Pyramids
3. Angkor Wat
4. Panama Canal
5. Sydney Opera House
6. Taj Mahal
7. Mount Rushmore
8. Trans-Siberian Railroad
9. Machu Picchu
10. Golden Gate Bridge

Ultimate Ten Engineering Triumphs
These could qualify as a more up-to-date list of the Wonders of the Modern World.

1. Space Station Alpha : the international space station to be completed in 2006
2. Akashi Kaikyo Bridge : the longest suspension bridge in world (2.5 miles), linking Kobe and Awaji in Japan
3. Biosphere 2 : a self-contained miniature version of the planet Earth in Arizona
4. CERN's Giant Collider : the world's largest scientific instrument, seven miles in circumference
5. Itaipu Dam : the world's largest, on the Piranha River between Brazil and Paraguay
6. Aquarius : underwater research laboratory near Florida
7. Petronas Towers : in Kuala Lampur, Malaysia, the tallest buildings in the world (Note: in 2003, 1483 foot tall Petronas Towers were surpassed by the 1667 foot Taipei 101 tower in Taiwan. In 2007, the still-under-construction Burj Dubai in the United Arab Emirates reached 1,921 feet and is expected to reach over 2,275 feet when finished in 2008. In the related category of "tallest free standing structure" (whatever that means) are the Indos Telecom Tower in Jakarta, Indonesia (under-construction, 1,831 ft), the CN Tower in Toronto, Canada (1,815), the Ostankino Tower in Moscow, Russia, (1,772), the Xi'an Broadcasting, Telephone & TV Tower in Xi'an, China (1,542), and the Oriental Pearl TV Tower in Shanghai, China (1,535). The tallest building in the United States is the Sears Tower in Chicago (1,450), but the proposed new World Trade Center in New York will stand 1776 feet.
8. Oresund Fixed Link : ten mile bridge and tunnel connecting Denmark and Sweden
9. Airships : the rebirth of zeppelins
10. Toronto Sky Dome : a baseball stadium with a retractable roof

1. Great Wall of China
2. Lost City of Petra : Jordan
3. Statue of Christ the Redeemer : Rio de Janeiro, Brazil
4. Machu Picchu : Peru
5. Chichén Itzá : Mexico
6. Roman Colosseum : Italy
7. Taj Mahal : India