By a roundabout route I have come across this useful collection of free web tools from University of Mississippi Library which could be used by us in teaching students how to choose their topic. One of the must cover elements of my teaching (facilitating student learning, sorry!) is use of keywords - defining what it is that is being researched.
The Google Wonder Wheel is one that I should have known about before. Recommend you try this. It's not infallible in the links it makes, but it's easy enough to find and may engage our users - the ones that think visually, that is!