Why One Tiny Island is Still a Domain Name Giant (zdnet.com)
The small Pacific island Tokelau is still the most populated country-level domain in the world, outnumbering the 20.8 million domains that use China's .cn. From a report: UK registry for .uk domains has published its latest topsy-turvy map of the world, with land mass weighted according to the number of registered country-level domains. As it was two years ago, Tokelau remains the world's 'largest' country, thanks to its free registration policy; the number of .tk domains reaching nine million in 2012 and from there tripling to 31 million by 2016. Today, the number of .tk domains stands at 21.2 million, but it still remains the largest, just ahead of China. The number of domains with China's .cn has increased over the past two years from 17 million to 20.8 million, making it the second most widely used country-level domain in the world.
Part of the free deal is you need a minimum of of unique hits a month (on which they serve an ad over), or you lose the URL.
After reading the article (which just repeats legend from the map), I still does not know why .tk ccTLD is successful.
This is the page you are looking for.
http://www.tcl.tk/
Niue's TLD .nu became more popular in Scandinavia, as nu means "now" in Swedish, and it was open to individuals before Sweden's own .se was. At one time, many erotic sites for the francophone market were in .nu, as nu means "naked" in French.