itch.io

Update. These links courtesy of user Terran:

Some standalone programs for making chiptune are: 

• FamiTracker: http://famitracker.com/ 
• MilkyTracker: https://milkytracker.org/ 
• Pxtone: https://www.cavestory.org/pixels-works/pxtone-collage.php

Some VSTs if you want to use a DAW: 

• https://transverseaudio.com/posts/best-free-chiptune-vst-synths 
• https://www.tweakbench.com/ 
• https://woolyss.com/chipmusic-plugins.php