Japan Ministry of Education, Culture, Sports and Science publishes daily measurements of radioactivity across the country. Levels of national radioactivity by region can see seen here: http://atmc.jp/ (without the www). There’s a language translation setting near the top of the page. By clicking on each graph you can see more detailed information.

Other measurements are available for water: http://atmc.jp/water/

And rain: http://atmc.jp/ame/

Since March 27, they have been publishing levels of radioactivity in water in Fukushima Daiichi nuclear power plant, for each unit separately: http://atmc.jp/water_plant/

The data is updated daily.

Another more detailed graph for Tokyo area (measured at Shinjuku) is available here:
http://mextrad.blob.core.windows.net/page/13_Tokyo_en.html

For better understanding: the graph shows values from 0 to 5.0 microSv/h. Japanese law states that competent authorities must be notified when the value reaches 5.0 microSv/h or more.

Some more info…

According to Japanese standards, levels of radioactivity in water which are considered unsafe, begin at more than 300 Bq/kg for Iodine-131, and more than 200 Bq/kg for Cesium-137. These levels actually vary by country — for instance, in some EU countries, levels considered unsafe start at 600 Bq/kg or more.

About 15,000 Bq of Iodine-131 is equivalent to 0.33 mSv (milisieverts) or 300 microSv which is about half of a stomach x-ray. (Source: SaitamaAJET).