We removed our free Sandbox April 25th.
You can read more on our blog.

Opa

Warning

Since the last version of OPA, the current OPA service is deprecated and will be removed soon. Meanwhile you’re invited to use the following custom service which support the last version of the OPA language.

https://github.com/dotcloud/opa-on-dotcloud

To use it, open a terminal and run the following commands:

$ git clone git://github.com/dotcloud/opa-on-dotcloud.git
$ dotcloud create myapp
$ dotcloud push myapp opa-on-dotcloud

The cloned repository contains an application example called “app.opa” which you can replace by your own OPA code.