Can you recommend a free compiler/IDE for writing some simple Io programs?
I want to learn the language at home in my spare time.
You should be able to download the compiler and runtime environment from the link you provided. I don't think there are any pre-packaged versions of the compiler/runtime environment , so you'll likely have to build/compile it yourself. Check out the documentation here: http://www.iolanguage.com/scm/io/docs/IoGuide.html for build/install instructions.
AFAIK, IO doesn't currently have a nice graphic UI like you might see in Smalltalk or Self. You'll most likely find yourself editing scripts in your favorite text editor, and then running them from the command line.