MMSSTV - By far the most popular software used to send and receive SSTV messages.
RX-SSTV - Simple software that recieves SSTV messages.
EasyPal - A software for sending "digital SSTV" messages. Digital is nowhere near as popular as standard analog SSTV, but is very cool to play around with if you can find people willing to make contacts with you on it.
SSTV-Ftp - A basic program that is used to upload SSTV image history to the web. This same developer has several other bits of SSTV related software on their website.
CQSSTV - Filled with lots of SSTV feeds.
World SSTV Cams - Another site filled with SSTV feeds. This is my preferred site for checking to see how far my images reach.
Pixlr Editor - Online tool good for resizing, adding text, and doing whatever you may need to do to prepare an image to be added to your gallery of SSTV images that you send.
WSJT-X - Very popular software containing lots of popular digital modes, most notably, FT-8 and FT-4.
FLDIGI - Another software that supports digital modes like RTTY and PSK.
RadioID.net - All things DMR and NXDN.
DMR For Dummies - Another good DMR resource.
ARRL Band Plan Site - Official ARRL band plan website.
ARRL Amateur Radio Bands Chart - Shows all of the frequencies available for use by the ham radio community, and then also which operatior classes are allowed to use which frequencies.
Bandplans.com - Large unofficial community made band plan database. You can cross reference this with other websites to make sure you are transmitting on the right frequencies.
RepeaterBook - Do a proximity search and find your local repeaters.
CHIRP - Program your radio with CHIRP.
Ham Radio Crash Course
Ham Radio 2.0
David Castler
HamRadioConcepts
Kevin Loughin
TheSmokinApe
Ham Radio Dude
Jim W6LG
Note: Theres so many creators that it is near impossible to list them all. I highly encourage you to search around on YouTube, and subscribe to the creators that you find informative and enjoyable to watch.