A WebSDR is a Software-Defined Radio receiver connected to the internet, allowing many listeners to listen and tune it simultaneously. SDR technology makes it possible that all listeners tune independently, and thus listen to different signals; this is in contrast to the many classical receivers that are already available via the internet.
More background information is available here. Questions and comments can be sent to PA3FWM, the author of the WebSDR software and maintainer of this site; but please check the frequently asked questions first.
WebSDR servers can register themselves automatically on this site, leading to the below list of currently active WebSDR servers.
Since Java version 7u51, Java applets need to be enabled for each site separately; see http://websdr.org/java.html for instructions.
Currently there are 117 servers active, with 630 users and 284 MHz of radio spectrum.