I co-wrote (with Steve) SoundConverter for Windows:

http://dekorte.com/projects/shareware/SoundConverter/

The interface talks to an Io "server" which handles business logic.

We're currently working on a shopping aggregation startup. The crawler is written in Io.

Pixar's RenderMan image tool uses Io as its scripting language:

https://renderman.pixar.com/products/tools/it.html

Io is still under development so you might run into the occasional bug. Bugs are encountered more often in the addons than in the core interpreter (which is stable).