Here are the links that I find useful.
What is Amateur Radio?
Nice leaflet about Amateur Radio - http://www.larc.org.uk/wp-content/uploads/2017/04/Amateur-Radio-Leaflet.pdf
What is Amateur Radio (youtube) - https://youtu.be/8x6x_6mDVlQ
What is Amateur Radio (youtube) - http://youtu.be/ieM0nY1mokE
Antennas:
Delta
Rick DJ0IP 40m Delta Antnna - http://www.dj0ip.de/my-favorite-antennas/my-favorite-40m/
About Loop Antennas - IW5EDI - http://www.iw5edi.com/ham-radio/2599/about-loop-antenna
(Polish) - Delta wg SP7LA - http://sp5mxf.com/antena-delta-wedlug-sp7la/
The Loop Revisited - https://www.eham.net/articles/21273
EFHW
SP8QEP Project (Polish language) - https://sp8qep.wordpress.com/
Multi Band End Fed Antenna - https://www.nonstopsystems.com/radio/frank_radio_antenna_multiband_end-fed.htm
RaDAR – End Fed Half Wave Antennas - https://zs6bne.wordpress.com/2013/06/20/radar-end-fed-half-wave-antennas/
Bazooka
The Bazooka antenna by IW5EDI - http://www.iw5edi.com/technical-articles/the-bazooka-antenna
1/4 Vertical Bazooka for 40m - http://www.on8im.be/en/homemade/homebuilt-antennas/53-1-4-vertical-bazooka-antenna-for-the-40m.html
Magnetic Loops
An experimental eight-shaped loop for HF - http://f5ad.free.fr/Liens_coupes_ANT/G/PA9OK%20Boucle%20magnetique%20en%208.htm
Small Magnetic Loop (80&40M)- https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
Baluns
Enhanced OCF Dipole Balun 2014-07-10 OH7SV - http://www.saunalahti.fi/hohtola/ham/enhanced-OCF-balun/enhanced-OCF-balun.html
Baluns I - 1:1 current balun - http://vk6ysf.com/balun_guanella_current_1-1.htm
Baluns II - 1:1 Ruthroff voltage balun. - http://vk6ysf.com/balun_1-1.htm
Various
Interesting article about various construction of wire antennas : http://hflink.com/antenna/
Xloop Antenna - http://bamlog.com/xloop.htm
K9AY by SP3KEY - http://sp3key.com/klub/k9ay_pro/
5/8 Vertical for 20m - http://www.pa3hcm.nl/?p=902
IW5EDI Antennas - http://www.iw5edi.com/ham-radio/?hf-antennas,21
Introducing the "All-Band" Doublet - https://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htm
Feeding the Beast - https://www.donkeith.com/n4kc/article.php?p=13
All Bands Doublet - http://webclass.org/k5ijb/antennas/All-Band-Doublet.htm
ZS6BKW Antenna (G5RV) - https://pa0fri.home.xs4all.nl/Ant/ZS6BKW/zs6bkw%20eng.htm
Screwdriver Antenna - http://www.w5jgv.com/GS3_antenna/index.htm
Mobile Antennas - https://www.qsl.net/g3tso/Hombrew-Mobile%20Antennas.html
Vertical Dipole Antennas by F4BKV Vincent - https://www.f4bkv.net/antenna-vda.html
CW
Helvetia Telegraphy Club (HTC) - https://www.htc.ch/index.php/en/
The CW Operator's Club - http://www.cwops.org/
Learn Morse Code (CW) Online - http://lcwo.net/
Titawa - http://titawa.pl/home/
First Class CW Operators' Club - https://www.g4foc.org/
Reverse Beacon Network - http://www.reversebeacon.net/
Activity of HB9EGA in RBN - https://foc.dj1yfk.de/activity/HB9EGA
Morsum Magnificant - http://www.n7cfo.com/tgph/Dwnlds/mm/mm.htm
Daily Quotes in Morse Code - http://www.morseresource.com/quotes.php
Home Made
30-Poles of Microcontroller Driven Analog Filter Fun - https://ac0c.com/main/page_homebrew_the_ultimate_audio_filter.html
SOTA
Summits on the Air - https://www.sota.org.uk/
SOTA Watch - http://www.sotawatch.org/
ICOM 7300
Spectrum display in N1MM from IC-7300 - https://www.youtube.com/watch?v=nasSxs_ueK4&t=312s
Other
Remote QTH - https://remoteqth.com/open-interface.php
DX Cluster DX Watch - https://www.dxwatch.com/
DX Maps - https://www.dxmaps.com/spots/mapg.php
VOCAP - http://www.voacap.com/