ACLED collects real-time data on the locations, dates, actors, fatalities, and types of all reported political violence and protest events across Africa, the Middle East, Latin America & the Caribbean, East Asia, South Asia, Southeast Asia, Central Asia & the Caucasus, Europe, and the United States of America.

A self-hosted network visualization on a 3D globe with support for IPFIX, Netflow and sFlow. Yes, it's a pew pew map. Consists of a back-end server and a front-end dynamic globe.

An air quality monitoring network built on a new generation of Internet of Things sensors. Using a new generation of laser particle counters to provide real-time measurement of PM1.0, PM2.5 and PM10. PurpleAir sensors are easy to install and only require a power outlet and WiFi. They use WiFi to report in real time to the PurpleAir map. Sells air quality sensors that are pretty expensive. Don't know if it'd be possible to upload data from other kinds of sensors.

Account creation requires authenticating with a Google account only.

They have a JSON API but it's read-only and just a straight dump from their database: https://www.purpleair.com/json

https://www2.purpleair.com/community/faq#hc-access-the-json

There is also a Thingspeak API: https://www2.purpleair.com/community/faq#hc-thingspeak-api

They also seem to have a REST API but you need an API key and ChannelID. Not a big deal, really.

I don't know if this service is suitable for my purposes.

The Internet Weather Map™ (IWM) is a free service that maps latency on the Internet. As an Open Community project, it uses data from volunteers all over the world to feed back latency on the Internet into a central database. Then that data is aggregated, and displayed in table and map formats, allowing you to see how the fast sections of the Internet are running. While it practically not possible to map out every segment of the entire Internet, the IWM product traces tens of thousands of segments to give you an informed idea as to it's overall latency.

The Latency Map is the heart of the service, which displays a map of any delays on the Internet, as well as in a table format. While the table displays the slowest segments, the map normally only displays delays (latency over 300ms). The data on this tab will refresh every 60 seconds, so there is no need to re-load the page manually.

Tools allows you get information on your domain name, including an MX Record Lookup, and some additional diagnostics.

A crowdsourced map of the weird and uncanny.

A web application (personal and otherwise) for tracking high altitude balloons, radiosonde beacons, and so forth on a world map. Desktop and mobile friendly.

Github: https://github.com/rossengeorgiev/habitat-mobile-tracker

A worldwide, real time, community collaborative lightning location network.

Might be an API. Data could be useful.

API documentation for wigle.net.

An interactive map of ham radio repeaters around the world, searchable by callsign, coordinates, tags, state, or relative directions.

An interactive map of the submarine telecommunications cables which form the inter-continental links of the Internet.

An interactive map of known pulsars, looking at Mutter's Spiral edge-on. You can scroll around and change the distances. Also shows gammaray bursts, supernovas,

A relatively simple yet useful online pulsar map generator. Displays the Pioneer and Voyager map by default but can generate custom maps with a bit of fiddling.

A largely browser based diagram and chart webapp written in Javascript. Use Ant to compile it, then zip it up and upload it to a web server. You can even use Python's BaseHTTPServer to play with it. Forces HTTPS. I was able to build it on my laptop pretty easily. Gives you the option to save to a number of different storage back-ends if you install the back-end webapp, including your local machine.

A Python module that lets you take data and easily generate a Leaflet map of it. Defaults to Open Streetmap but can also use Stamen Terrain and Toner, Mapbox Bright and Control Room. Can use custom tilesets if you have the appropriate API key. You can output the map to local storage. If you can define a map some other way, you can do the same with Folium (probably more easily). Polygons can be drawn on the map as you'd expect.

A map dataset in the public domain. Multiple scales of resolution available (1:10m, 1:50m, 1:110 million). Vector and raster images. Incorporates GIS attributes for mapmaking. Designed to make it easy to develop maps on your own. Comes as ArcMap MXD and QGIS documents. You can also download entire databases (even SQLite) all in one go.

Nominatim is the OpenStreetmap server for searching the database for an address and figuring out where in all of the map datapoints that address is. When you figure out where in the database it is, you can then serve up the appropriate map tiles. Powers the search box on the website. Used to import the map data.

A personal webapp for tracking your own location. Lets you set up a private location diary or a realtime tracker to share with people you give access to. A phone app for iOS and android is the user front-end. The app can publish GPS coordinates and your phone's status to an arbitrary HTTP API endpoint or MQTT broker if you tell it to. Your location gets plotted on a map. The back-end is called Recorder and is found on github: https://github.com/owntracks/recorder

An interactive map of the Linux kernel that runs in your browser. From what I can tell, it uses HTML and JavaScript.