Several organizations post market-share-ranked lists of domain name registrars and numbers of domains registered at each. The published lists differ in which top-level domains (TLDs) they use; in the frequency of updates; and in whether their basic data is absolute numbers provided by registries, or daily changes derived from Zone files.
The lists appear to all use at most 16 publicly available generic TLDs (gTLDs) that existed as of December 2009, plus .us. A February 2010 ICANN zone file access concept paper explains that most country code TLD (ccTLD) registries stopped providing zone files in 2003, citing abuse.
Monthly (but with approximately a three-month delay), ICANN posts reports created by the registries of 16 gTLDs. These reports list absolute numbers of domains registered with each ICANN-accredited registrar.
Monthly (but with three-month delay as it relies on ICANN data.) Dotandco.net publishes a list of registrars by volume.
Yearly (but covering only the period from 2002 to 2007), DomainTools.com, operated by Name Intelligence, Inc., published registrar statistics. Totals included .com, .net, .org, .info, .biz and .us. It cites "daily changes" (presumably from daily zone files) as the basis for its yearly aggregates, although it only lists quarterly changes.