CW beacons are traditionally used for propagation observation. Hearing a beacon means that there is propagation and you can probably make contact on the band of the beacon. 10m has over 100 kHz of band spectrum (28200 - 28300 kHz) allocated to CW beacons. 10m CW Beacon operation and beacon spotting is still quite popular among hams around the world, but WSPR and other more efficient modes are being used as well nowadays. Many other HF beacons exist in other bands in the designated beacon frequency sub-bands.
In 1979 the first IBP beacon went into operation. The innovation was that each IBP beacon would be cycling a range of frequencies in a time-synchronised global operation:
Beacon users can standby on any of these frequencies and with great likelihood receive one or more of the IBP beacons within just a minute or two. The frequencies are CW mode frequencies (not USB).
Naturally, beacon frequencies need to be clear and avoided for other ham TX operation. During contests and also otherwise, the beacon frequencies are often suffering QRM but by and large the IBP beacons can still be received well. IBP beacon operation requires maintenance and it is for that reason that some IBP beacons are temporarily off the air. The IBP beacon network status can be checked here:
VarAC contributes favourably with its beacons and makes it easy to check propagation in an elegant and efficient way. You can even use the "Callsign history" to check VarAC beacon SNR data over time. (During a QSO, the new "Verbose SNR" feature allows us to see fine SNR graphs.)
As responsible VarAC operators it is upon us to be cautious and protect HF beacon frequencies.
National and global beacon lists are available on the net.