freeware
feelings game (Matthew Keff)
"a sounds and physics sandbox toy" - Author's description Download on itch.io (Windows, Mac)
sandbox
radical rockits
Radical Rockits (RageSquid)
"a physics jetpack sandbox without any objectives." - Author's description Download on itch.io (Windows, Mac)
browser
Vilmonic (Bludgeonsoft)
"an artificial life and evolution simulator sandbox game that takes place in a lonely distant future." - Author's description Play on itch.io (Browser) [Via @Bludgeonsoft]