Some excellent free Linux software for Ambisonics and audio measurements can be found at Fons Adriaensen's website http://kokkinizita.net.

Guides to using Ambisonic with DAWs, special attention on Nuendo and Cubase.
Ambisonic VST and Audio Unit plug-ins for Mac OS X, PPC and Intel.

Aristotel Digenis has produced some VST plugins for Ambisonic Decoding as well as some Plogue Bidule extensions:

http://www.digenis.co.uk/plugins.html

A Composition/Performance Environment for Sharing Live Music Technologies

A variety of Ambisonic Decoders that are available for download.