A good primer on HF propagation.
Current geophysical data. Todays' SSN's/Fluxes
W6ELProp (Easy to use and information is easier to read.)
Windows-95/98/ME/NT/2000/XP 32-bit HF Propagation Models from NTIA/ITS (Complex, but the standard. The technical manual has an in-depth section on propagation theory.)
Radio Mobile (Not for HF and not exactly propagation, but very useful for terrain and system analysis.)
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