If you're looking for a tool to do either mind-mapping or traditional whiteboarding, Pixelglow Software's $10 Instaviz is a strong option.

For more traditional whiteboard sketching, give Avici Software's $5 Whiteboard HD a look. It gives you controls over labeling, provides a grid background to help in placement, and supports image inclusion from the Photos app. It also works with an iPad VGA adapter for presentation on a full-size monitor.

Diagramming and charting

For sophisticated diagramming of process charts, website wireframes, or other complex relationships, the Omni Group's $50 OmniGraffle is highly regarded. It supports freehand drawing but also has a bevy of intelligent object formatting and page layout controls. Its files are compatible with the desktop version, and you can export PDF view-only versions as well.

The $15 OmniGraphSketcher is good for traditional diagramming, such as for creating precision graphs in architecture, finance, engineering, and space planning.