Here are 10 sites that offer free domain registration for a limited time, usually for one year:
Freenom - Provides free domains with extensions like .tk, .ml, .ga, .cf, and .gq. Freenom
InfinityFree - Offers a free subdomain with its hosting service, but you can upgrade to purchase a custom domain. InfinityFree
Dot.tk - Specializes in free .tk domains. Dot.tk
AwardSpace - Provides a free domain with its hosting plan, but there are limitations on usage. AwardSpace
Webnode - Offers a free subdomain with its free hosting plan. Webnode
Weebly - Provides a free subdomain with its hosting plan, with options to buy a custom domain. Weebly
Wix - Offers a free subdomain with its hosting plan, and you can purchase a custom domain if desired. Wix
WordPress.com - Provides a free subdomain, with options to register a custom domain. WordPress.com
GitHub Pages - Offers a free subdomain if you use GitHub to host your site. GitHub Pages
000webhost - Provides a free subdomain with its hosting service, with options to purchase a custom domain. 000webhost
Make sure to check the terms and conditions of each site as there may be restrictions or specific requirements for using free domains.