http://www.1111111111111111111111111...111111111.com/

http://www.llanfairpwllgwyngyllgoger...ogogoch.co.uk/

http://www.thelongestdomainnameinthe...reandmore.com/

http://3.141592653589793238462643383...974944592.com/
it seems the subdomain of
http://14159265358979323846264338327...974944592.com/