A simple analysis of http://www.iana.org/domains/root/db
$ curl -s http://www.iana.org/domains/root/db | xmllint --html --xpath "//table/tbody/tr/td[1]/span/a|//table/tbody/tr/td[2]|//table/tbody/tr/td[3]" - 2>/dev/null | tr -d '\n' | sed 's/<a[^>]*>/\n/g' | sed -e 's/<[^>]*>/|/g' | awk -F '|' '{print $5,"|",$3}' | sort | uniq -c | sort -rn | awk '{if ($1>4) print $0}'
51 Amazon Registry Services, Inc. | generic
43 Charleston Road Registry Inc. | generic
22 United TLD Holdco Ltd. | generic
22 Uniregistry, Corp. | generic
18 Top Level Domain Holdings Limited | generic
14 Not assigned | country-code
14 Afilias plc | generic
11 VeriSign Sarl | generic
11 United TLD Holdco, Ltd | generic
11 Lifestyle Domain Holdings, Inc. | generic
11 Internet Assigned Numbers Authority | test
11 Dish DBS Corporation | generic
10 Richemont DNS Inc. | generic
8 National Internet Exchange of India | country-code
8 Microsoft Corporation | generic
7 The TJX Companies, Inc. | generic
7 Johnson Shareholdings, Inc. | generic
6 XYZ.COM LLC | generic
6 Public Interest Registry | generic
6 Kerry Trading Co. Limited | generic
6 Fiat Chrysler Automobiles N.V. | generic
6 FCA US LLC. | generic
6 Association Française pour le Nommage Internet en Coopération (A.F.N.I.C.) | country-code
5 Minds + Machines Group Limited | generic
5 Asia Green IT System Bilgisayar San. ve Tic. Ltd. Sti. | generic