Visual aids significantly improve the interest of a presentation. However, they must be relevant to what you want to say. A careless design or use of a slide can simply get in the way of the presentation. What you use depends on the type of talk you are giving.
Here are some possibilities:
*) Overhead projection transparencies (OHPs)
*) 35mm slides
*) Computer projection (Powerpoint, applications such as Excel, etc)
*) Video, and film,
*) Real objects - either handled from the speaker's bench or passed around
*) Flip~chart or blackboard - possibly used as a 'scratch-pad' to expand on a point