Sendmail is the MTA that takes care of the biggest share of E-mail transportation these days. It was originally written by Eric Allman, who co-founded the Sendmail company.
It was written before the Internet E-mail and SMTP came along. It has support for, well, just about every E-mail transportation method imaginable, past and present - before the standards, you never knew what kind of places you needed to get that mail into. If you knew how to touch the relatively obscure sendmail.cf, you could easily get it to transport mail to, say, UUCP or get mail through SMTP-gateway (internal.address%gateway@site.example.com).
These days, such flexibility is no more that much required, but Sendmail is still around, mostly because people no longer need to write sendmail.cf (the advent of M4 macro parsing has made Sendmail configuration as easy as writing a few lines and saying "bake it so") - simply, it's much easier to use and configure than it used to - and, ah, it's the standard. =)
These days, the open source (BSD-licensed) Sendmail is at version 8. A commercial version of Sendmail also exists, from Sendmail, inc.
Sendmail is very popular, but other MTAs, such as Postfix and qmail, are getting a tighter stranglehold...