PowerPoint: A popular, easy-to-use option. (Advice for creating a poster with PowerPoint).
Adobe Illustrator, Photoshop and InDesign: Professional software including lots of high-resolution images, but are more complex and expensive.
Open Source Alternatives: OpenOffice - free alternative to MS Office (Impress is its PowerPoint alternative). Inkscape & Gimp Adobe product alternatives. For charts and diagrams, Gliffy or Lovely Charts. A complete list of free graphics software.