This is crying out to be done like the visual word map in AquaBrowser, but here's a browseable tag cloud based on data from nearly 2 million keyword searches on our OPAC.
The code looks for other keywords that were entered as part of the same search (e.g. "ethics of nursing care") to draw out the most commonly used words. For example, the most common keyword used with "performance" is "management". The size of the word in the cloud is determined by how often it appears with the search keyword.
I've not removed keywords that generated zero search results, so the cloud for "acrobat" includes "abode". (I've now removed zero result searches)
I'll have to have a play to see if there's a way of incorporating the cloud into the OPAC — for example, if you used a vague/general keyword such as "health", then maybe the OPAC could suggest more specific searches for "health care", "mental health" or "health promotion"?