Graphs or pie charts come to mind when one thinks of visual aids, but many more kinds of visual aids are out there.
Exploratree’s readymade thinking guides can also work as a source of visual aids for presenting concepts and ideas.
Visual thinking archive by Armano which can be used as visuals for presentations and slideshows
Some ideas for concept maps, process charts.
PMI, SWOT and other thinking maps, Y chart
Thinking maps – circle map, bubble map, brace map, bridge map etc
Thinking maps templates-circle, bubble, brace, flow, and tree map
Amazon books:
Visual tools for constructing knowledge (David Hyerle)
Visual Tools for Transforming Information Into Knowledge
The “Frequently bought together” section of the above book provides even more relevant book choices.