Software to use for creating posters for academic conferences?

I would recommend Inkscape:

• vector graphics
• powerful
• free
• intuitive
• cross platform

I haven't written a poster yet, but if you're a Latex savvy, there are plenty of packages that allow you to design posters.

See related questions:

• "How to create posters using LaTeX"
• "What can you tell me about poster design and typography in LaTeX?"
• "Conference Announcement Poster"

Powerpoint. There are hundreds of PowerPoint poster templates available online, many of them are good, and it is one of the standard formats people accept — and can be exported to PDF for easy post-conference distribution.