I haven't used it in ~5 years, but Sphinx (http://sphinxsearch.com) might be a reasonable competitor. Back then it was pretty pleasant to use...especially it's indexing abilities, which were significantly faster than ES. I can't speak to it's distributed ability since our index was never that large, but the documentation purports to have that capability.

Sphinx has some curious features, like being able to run as a MySQL storage engine (no doubt the result of the author being ex-MySQL), but it's also got most of the features that you come to expect in search engine, so I think it's fair to call it a competitor. That said, we still use ES.