A really freakin’ big earthquake just hit Tokyo — strong enough to make tall buildings visibly shake around. My own office building (brand-new, finished last year) is still swaying like a ship on the high seas 20 minutes later. The force of the quake was enough to knock over one of the monitors on my desk.
From what I can see from the 17th floor, there is no major damage, though some smaller buildings have been evacuated and what look to be emergency response helicopters are circling overhead.
The quake was magnitude 7.9 centered in Tohoku. Nikkei is currently reporting that a tsunami of up to 6 meters is possible in that area, and USGS is warning of tsunami effects throughout the North Pacific as far as the west coast of North America. Hope our readers are OK.

[Edit by Roy below] People outside of Japan and/or without access to a TV should try Al Jazeera International’s high quality free stream to see the latest. They’re basically broadcasting the footage from NHK with experts at the USGS and such doing voiceover.
The get an idea of how massive this quake was, here in Kyoto – more than halfway across Honshu from the center, I felt my house shake for something like two minutes, although it was rather gentle and nothing even fell over. But even though it was not very strong here, it was still easily the longest quake I have ever experienced.
Please post your own personal observations or links to good live news sources in the comment thread.
[Update by Roy @5:00 pm]
Here is the Japanese Meteorological Agency tsunami warning map.

All place names that do not specify refer to the entire prefecture.
Regions with LARGE tsunami risk (red) are as follows:
Iwate, Miyagi, Fukuoka, Hokkaido Pacific coast central area, Aomori Pacific coast, Ibaraki, Chiba Kujukuri outskirts, Izu islands, Hokkaido Pacific East Coast, Hokkaido Pacific West Coast, Aomori Sea of Japan coast, Chiba interior, Ogasawara Islands, Sagami Bay and Miura Peninsula, Shizuoka, Wakayama, Tokushima
Regions with REGULAR tsunami risk (orange) are as follows:
Aichi outer sea, Mie southern area, Kochi, Miyazaki, Tanegashima/Yakushima region, Amami/Tokara islands, Hokkaido Sea of Japan southern coast, Mutsu Bay, Tokyo Bay inner bay, Ise, Mika Bay, Awaji southern region, Ehime Uwakai coast, Oita Seto Inland Sea coast, Oita Bungo Channel, Kagoshima East and West regions, Okinawa main island, Daito Islands, Miyakojima, Yaeyama Islands.
Regions with LOW level alert (yellow) are as follows:
Sea of Okhostsk, Osaka, Hyogo Seto Inland Sea coast, Okayama, Kagawa, Ehime Seto Inland Sea coast, Ariake/Yatsushiro Sea, Nagasaki western region, Kumamoto Amakusanada coast, Akita, Yamagata, Niigata Kaminaka and Kaetsuchi, Sado region, Toyama, Ishikawa Noto region, Hiroshima, Yamaguchi Seto Inland Sea coast.
Other regions do NOT have a tsunami warning or watch at this time.
[Update at 5:20]
I just heard that the Philippines raised their tsunami alert level to the highest in many years, NE coast, which faces Japan, will be hit in around 2 hours.
Here is a map from the US National Oceanic and Atmospheric Administration (NOAA) showing travel times for the tsunami to arrive at various areas, starting from the origin time of 2:45pm.

[Update@5:30pm]
Areas in the Philippines with a level 2 (which is high) tsunami warning are as follows. Will start to hit between 5pm and 7pm local time. (From Inquirer.net)
• Batanes Group of Islands
• Cagayan
• Ilocos Norte
• Isabela
• Quezon
• Aurora
• Camarines Norte
• Camarines Sur
• Albay
• Catanduanes
• Sorsogon
• Northern Samar
• Eastern Samar
• Leyte
• Southern Leyte
• Surigao del Norte
• Surigao del Sur
• Davao Oriental
• Davao del Sur
Marianas islands and Russian Pacific coast also under high alert, Various Pacific island territories of Guam, Taiwan, the Marshall Islands, Indonesia, Papua New Guinea, Nauru, Micronesia and Hawaii are under a lower tsunami watch.