I have search for open source SaaS Application boilerplate for developer and found two interesting Open source project which will save your lots of time.
- https://craterapp.com/ Complete Invoicing Solution
Crater is an open-source web & mobile app that helps you track expenses, payments & create professional invoices & estimates.
Web Application is made using Laravel & VueJS while the Mobile Apps are built using React Native.
Crater Provides Mobile application as well.
Wave is the ultimate Software as a Service Starter Kit packed with awesome features like authentication, subscriptions, invoices, announcements, user profiles, and so much more!