I've gushed a lot about Guard. It's really transformed my workflow, and I love using it for TDD. I've been reading Seven Languages in Seven Weeks and going through exercises in Io, and I wanted to get some TDD-style feedback:

watch -n1 io two_dim_list.io

This will run the command io two_dim_list.io every second (in this case my test file).

If you're using OSX, you can install watch with Homebrew:

brew install watch