Have a look at the popular KiwiSDR
SDR stands for software defined radio which uses software for many of the functions previously done by hardware in a radio. Most modern amateur radios are software defined radios, like my FT-710 HF radio. WebSDR is an SDR which is connected to the internet, which multiple users can use simultaneously to tune in to different frequencies, using a browser interface and listen to using computers or mobile phones. KiwiSDR was designed by ZL4VO/KF6VO. KiwiSDR is a wonderful piece of equipment which can tune from 0-30 MHz. When I checked, there were 859 such receivers online and 1020 people were using them.
It can receive and decode a wide range of signals. KiwiSDR runs on a Linux based Beaglebone computer, with additional radio circuits. KiwiSDR as the name implies, was designed and built in New Zealand. They are plug and play devices which do not need another computer. KiwiSDR 2 comes with BeagleBone Green, in aluminium enclosure. Plenty of holes in the enclosure ensures good cooling and permits use of an external fan for cooling if needed.
Shipment includes GPS antenna and self test cable. The supplied SD card should not be inserted and is only for re-flashing the BeagleBone Green in case the Debian filesystem becomes corrupted. Kiwi configuration will be lost with a re-flash.
We have to plug in a low noise 5V power supply, an Ethernet cable to the internet enabled network and an HF antenna. Then it is accessible from anywhere in the world on the internet browser. DC power supply is through a standard 5.5mm round connector with 2.1 mm center hole and should be rated at 5V 2A and of excellent quality. They have suggested genuine Apple USB iPhone chargers as a good power supply.
For amateur radio enthusiasts, KiwiSDR can bring you AM, FM, USB and LSB voice signals from around the world. It can also decode popular digital modes FT4, FT8 and WSPR. KiwiSDR can also decode CW or Morse Code signals in amateur radio. Utility of KiwiSDR is not limited to amateur radio or ham radio signals. It can decode and display weather FAX signals and DRM (Digital Radio Mondiale) digital broadcast. Stations of Very Low Frequency (VLF), Long Wave, Medium Wave, and Short Wave can be listened to on them. There are a few more radio signals which it can pick up as well.
Just like my FT-710 radio, the KiwiSDR also has a spectrum scope and waterfall. I used to listen to amateur radio stations using KiwiSDRs available online before I bought my radio. You need only a computer connected to the internet and no antenna or radio is needed. Many of my ham friends use WebSDRs to find out the reach of their signals and also to check the quality of transmitted audio signals. I have used it sometimes to compare the noise levels from another nearby location, as my location has upto 9+ and more of noise on many bands. I have even tried split operation with RX from KiwiSDR and TX from my FT-710, though it is a bit tricky to time the pressing of the PTT and cutting the audio of my computer during a QSO with frequent change over.
Four users can simultaneously use a single KiwiSDR connected to the internet. Yesterday I saw a post on X mentioning that one ham has six SDRs with different types of antennas connected to the internet. Two of them were KiwiSDRs. Connecting more SDRs to the web allows more users to access radio signals reaching that location. Another very interesting aspect which I read is that multiple KiwiSDRs operating at different locations can be linked together to locate the source of an unknown radio transmitter with fair accuracy. The time difference in arrival of signals at the SDR allows this calculation, like the radiotriangulation used by amateur radio operators during fox hunts! Let me thank John, ZL4VO/KF6VO profusely for permitting me to use the images from http://kiwisdr.com/ and http://kiwisdr.nz/ for this post.