WSPRnet — The Weak Signal Propagation Reporter Network is a group of amateur radio operators who use open-source software to test propagation conditions using very low-power transmissions.
PSK Reporter by Philip Gladstone — Similar to WSPRnet but data is gathered by receiving the PSK, JT65 and FT-8 protocols. You don’t need a license to contribute data because the system is receive only.
Reverse Beacon Network — A network of volunteers listening to the bands and reporting what stations they hear, when and how well.
N6SJV’s Cali VHF UHF Page — Spotting network with a focus on Northern California and the Greater San Joaquin Valley. Sponsored by the Lodi ARC.
VHF/UHF Conditions Map — Maintained by the Mountain Lake, MN public school district.
Kurt Harnish’s Maps — Covers 2, 6 and 10 meters.
DXMAPS — QSO/SWL real-time information.
William Hepburn’s DX Information Centre — Lots of information on DX and more.
DXWatch — Hosts the Reverse Beacon Network. Rather than a map format, this site lets you configure filters and display DX spots in a scrolling list.
Space Weather — Your source for “What’s up in space.”
VOACAP — Voice of America Coverage Analysis Program. Free professional high-frequency (HF) propagation prediction software from NTIA/ITS.