What plugins may I add to my sandbox sites?

You may add any WordPress plugin you wish, whether it be from the WordPress Plugins Directory, or from private plugin creators.

Why? 

At WP Sandbox, we believe you should be able to do anything with your sandbox that you could with a stand-alone, self-hosted WordPress site.

We let you install whatever plugins and themes you like, and you can use SSH, WP-CLI, and SFTP to access the filesystem to edit the code as you see fit. And yes, you can even access the WordPress database for each of your sandboxes.