I threw this together to accompany my Producing Great Sound book.
It uses convolution modeling to create random room tone.
Load a clean, breath- and noise-free sample from the production track (15 frames or so).
The sample isn't repeated, just used as a model for timbre. So the output doesn't sound looped.
I've used this (or a similar process) on a bunch of projects over the past year or two. Sounds fine, even in a theatrical mix.
Free, open source VST or AU plug-in. Runs in Macs only, and requires you load the free open-source SonicBirth first. Full instructions with the download.
More info and download links at GreatSound.info/roomtone
Comments and modifications welcome.