Sky Wave Propagation

VOACAP - (Voice Of America Coverage Analysis Program)
VOACAP is HF propagation prediction software, and this version has a nicely designed and easy to use web interface:
https://www.voacap.com/hf/

prop.kc2g.com
Web interface that provides information about the ionosphere from ionosondes (beacons) around the world. MUF (Maximum Usable Frequency) and foF2 (critical frequency) amongst other things are displayed.
Again, nicely designed and easy to use web interface:
https://prop.kc2g.com/