Lightning Linter Service
This is just a web application wrapper around the actual Salesforce Lightning CLI.
This is not an official product of Salesforce, updates and maintenance are not guaranteed. It uses the same node package as the Heroku plugin, however, so it should be possible to update the important parts just by using npm update.
There is a public version at https://lightning-linter.herokuapp.com/. Again, that service is offered not as an official product and comes with no warranty.