Thanks for the suggestion. Something like PHPMyAdmin is so widely used that it would be an excellent candidate.
I've thought a little already about adding services which would provide an app ready to go — there are one or two I personally would love to have included in my own stacks (MailHog comes to mind). Although you're right, these web apps can be installed manually, the whole point of Indigo is so save devs spending time messing with their infrastructure!
That said, I have a vague nervousness about going down the path of providing web apps, as it could be asked, why not then also have a quick Wordpress installer etc etc But so long as we focus on web apps which do one thing — support the developer in their work — then I think we should be fine.