I could be wrong, but it doesn't seem like anyone willingly deploys new apps using mongrel anymore. It's all passenger, glassfish, etc these days.
Thin is built with the HTTP parsing code from Mongrel - and, curiously, has always admitted it.
A large number of the people who shout "Mongrel sucks" have those funny ways of saying "except when it doesn't" under their breath right after.
But yeah for a while, it was Mongrel or (ew) Webrick. How far we've come! :P