I believe ecomaps and genograms are really just specific kinds of
flowcharts.

Visio would be especially good for this -- or a non-MS product such as
SmartDraw (which makes very good-looking stuff).

If you want to do it in PowerPoint, I'd use the autoshapes on the Drawing
toolbar. Just use the regular shapes to draw what you want -- ovals,
rectangles, etc. -- click the tool on the toolbar, then either just click on
the slide or click and drag on the slide. Press Shift when you drag on the
slide to constrain the shapes to square/circle/etc. proportion.

Use the line tools on Draw | Autoshapes to add lines. You can use
"connectors" to add lines that connect one shape to another. When you move
one of the shapes, the line stays connected. (Sometimes these can be
difficult to work with, though, so it's often easier to stick with lines --  
just depends on the diagram.) Format them by selecting them, then using the
tools on the Drawing toolbar -- hover your mouse until you see the tool tips
saying "line style" (thickness) and "dash style" (dashed or solid).

You can also click the Draw button on the Drawing toolbar to get to the
align and distribute tools, which will help you line up objects on the
slide. The Grouping tools on the Draw button may help you as well.