WordSmith allows users to develop concordances, find keywords, and develop word lists from plain text files.
The following recipes on Methodi.ca use WordSmith.
The entry for WordSmith needs to be edited so that it doesn't show up in the Mac tools list. It is, for now, Windows only. I have used the software on Windows, and it's quite easy to use, but the only way to use it on a Mac would be to run it in a Windows virtual machine.