Lago

Lago handles the billing plumbing that makes SaaS companies tear their hair out: usage metering, subscription management, invoicing, payment orchestration, and revenue analytics. If you're building usage-based pricing (per API call, per seat, per compute hour), this is the open source engine that competes with Chargebee and Stripe Billing.

Self-hosting runs on Docker Compose with PostgreSQL. The API and frontend are separate services, which adds some initial complexity but keeps things modular. Event ingestion handles real-time usage tracking, aggregation rules turn raw events into billable metrics, and the invoicing engine generates PDFs. Integrations with NetSuite, Xero, Salesforce, and tax providers like Avalara cover the accounting side.

Solo founders can run the full billing stack for free. Growing startups avoid Stripe Billing's percentage-of-revenue pricing by self-hosting. Enterprise teams get Lago Cloud for managed infrastructure with pay-as-you-go pricing.

The catch: AGPL license. If you embed Lago into a SaaS product, the copyleft clause may require open-sourcing your application. Many companies end up on the paid cloud plan specifically to avoid that legal exposure.