Ive tried to find something that many major search engines use.

Is there anything that can be used as keyword suggestion tool? i.e. a tool / plugin / mod that comes up with relevant or similar keywords for a search that a visitor has made.

Possibly this then could also extend to be a suggestion tool for related keywords by category. With suggestions displayed on and by category. So that visitors see keywords related to the category they are viewing.

Any ideas? Other than adding keywords to the Category column...

Sounds like a good idea. Cannot recall ever seeing anything like this though. Not to say there isnt. Andy and a few others may have long memories on this one.