Wanting to scan my site and get tag suggestions

I’m helping a non-profit migrate their site to the WordPress platform. They have dozens of posts. Rather than manually reading everything and figuring out tags (not code/html tags, but taxonomy tags). I’d like to find a tool that will scan the posts and list the most frequently used words. Anyone know of such a free tool?

(This is probably in the wrong forum, but I cannot find a list of forums. Perhaps the design could be improved to make that easier.)