There is a list of measurement sites besides this one (that I know about). Feel free to add any missing info, or correct any errors/omissions as I have not thoroughly tried out every one. Note that this forum places a limit on the number of links, so they are all altered. In addition, the posting character limit means two postings are required.

h**p://radiationnetwork.com/
USA only, various GM tube sensors, map alerts, no historical data, real-time, free.
h**p://www.radmon.org/
world-wide, various GM tube sensors, user-set map/email alerts, historical graphs, real-time, free.
h**p://www.gmcmap.com/
world-wide, mostly GQ Electronics sensors but others allowed, no alerts, historical data tables, mostly real-time but old data mixed in, free.
h**p://eurdepweb.jrc.ec.europa.eu/EurdepMap/Default.aspx
Europe only, 4200 government stations from many countries (several sensor types), no alerts but very detailed detections, no graphs (I think), near-real-time (read the Disclaimer), free.
h**p://www2.epa.gov/radnet/near-real-time-and-laboratory-data-state
USA only, >100 government stations in every State (several sensor types), no alerts, historical graphs, near-real-time, free.
Note that this site, Netc.com, also logs and graphs the EPA's Beta and a Gamma range, but in a much more user-friendly way. Paying members can see the graphs, the gamma values, and reactor site names on the map.

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h**p://www.blackcatsystems.com/RadMap/map.html
USA and Europe, only Black Cat detectors, current readings only, real-time, free.
h**p://radioactiveathome.org/map/
World-wide, only project (kit) detectors that are occasionally available, no alerts, historical graphs, real-time, free, good reactor data via the map. The icons and graphs are also shown on the Radmon.org map.

It should be noted that most of these sites (as well as this one, Netc.com) have alot of useful information, either in the form of linked pages or in the respective user forums.

Interesting list, I had no idea there were so many out there.

I started as a user on the BlackCat system and then I moved to RadiationNetwork until I was introduced to NETC and what it can do.

I found another one; had seen it years ago but only now realized it has real-time public data on a map:
http://safecast.org/tilemap/?y=45.1&x=-93.8&z=4
It's world-wide, many sensor types, the green circles have real-time data with historical graphs, data downloads possible, no alarms (I think), free. It also shows single-measurement trails from mobile sensors with color coded values (check out the Chernobyl trail done in 2014, and Fukushima area). Right-click on the trail to see the nearby measurement details.

For readers not familiar with this site (Netc), here is a short description to compare with the above:
World-wide, many sensor types (both private and government sites), real-time data with historical graphs going back up to 5 years, admin-set alarms for map color and email alerts (including nearby sensors, not just one), free for current data and map icons, but subscription required to see the graphs and get email alerts.

The graphs and email alerts in my opinion make the subscription well worth it. Also being able to access the government data farther back than the EPA themselves allow, and in such a convenient way is frankly excellent design (thanks, Harlan/hey, and KC for moderating).

And another: http://www.uradmonitor.com/
World-wide, a few variations of their own calibrated sensor, real-time map data with graphs and email alerts, free. It aggregates overlapping sensor data in a nice way, separating it as you zoom in.