If you do a Search for an online CONCORDANCE for Tanach, chances are near certain you'll end up in a Christian site, which would be inappropriate, even if your search there can be successful. In fact, most such sites sport a non-denominational facade, so the searcher may well think he's on safe ground when in fact he thereby gives aid for trapping naive Jews who know next to nothing about their own religion. There are many Christian sites that set out a priori to hunt for and trap such innocent but clueless Jews. Is there a religious Jewish site (also as opposed to a "Reformist" brand) for searching Tanach we can rely on and explore comfortably, without otherwise abetting those who would will the Jew forsake his roots?
Mechon-Mamre provides a search box which, when you type in a Hebrew word, comes up with all the sources in the Tanach that have the word. I believe you can also do it with English too.
It's far from being exhaustive.