What software should I use to make a printout of something for a poster board?

Adobe Illustrator is pretty much the best software out there at the moment. This amazing piece of software can do anything for large printouts.

Microsoft Visio is great at managing large flowcharts. It is much like Visio, but for flowcharts.

Xara LX is an open-source (free) alternative to Illustrator and Visio.