Links

What is QRSS? - Here KA7OEI has provided a great write up on the principles of QRSS and links to software to decode QRSS
NT7S Arduino librairy code at GITHUB - A very good Arduino library for the SI5351 sig gen.
WA4DSY's online filter calculator - An online calculator for bandpass and lowpass filters
Minikits - An Australian componet supplier that specialises in RF and hard to get components, highly recommended
Tonne Software - An uncommon catalog of original and interesting electrical-engineering programs
Mini ring core calculator - a great coil calculator written by DL5SWB
Coil 32 - freeware software for coil design calculations
Extreme narrow bandwidth techniques - by ON7YD
ELSIE - The Windows program for electrical filter design and analysis
WSPR-net - The Weak Signal Propagation Reporter Network is a group of amateur radio operators using K1JT's MEPT_JT digital mode to probe radio frequency propagation conditions using very low power (QRP/QRPp) transmissions. The software is open source, and the data collected are available to the public through this site.
WSJT-X - web site for WSJT-X with good instructions
Atom - Fantastic text editor