Recently I was asked to investigate interference in the 2.4GHz ISM band at a particular location to ascertain which bands were most clear of spurious. The following plot is with max hold and using a ~0dBi antenna. The plot highlights the depth of the problem. Even the noise floor outside the ISM band (RHS) is 10dB above the noise floor of the analyser. Further measurements using a spectrograph showed the congestion through time and frequency. The interference severely limited the range of the radio link.
Some examples of what uses this ISM band are Bluetooth (802.15.1), Zigbee (802.15.4) and WiFi (802.11) – here are the frequencies used: