sdr.hu is a directory of software-defined radio receivers, run by András HA7ILM the author of the OpenWebRX software that drives those receivers. There are currently just over 500 receivers online, all around the world.
Recently the site has added a registration requirement, which restricts the site directory to individuals with a ham radio license and which re-emphasizes that the intent of the software is to support amateur radio activities. The OpenWebRX software is also announced to be no longer supported, due to a change in focus by the author, and also due to the end of service of the Python 2 language in which portions of the software are developed.
There are several other sites that keep similar lists of SDRs. kiwisdr.com, ve3sun, and linkfanel each keep directories of OpenWebRX and KiwiSDR receivers, and websdr.org lists sites that use the similar-but-different WebSDR software.
Originally noted at The SWLing Post.