Billing

4 open source tools compared. Sorted by stars. Scroll down for our analysis.

Tool	Stars	Velocity	Language	License	Score
Lago Open Source Metering and Usage Based Billing.	9.7k	+29/wk	Go	-	55
turboquant_plus	6.8k	+55/wk	Python	Apache License 2.0	77
Kill Bill Open-source subscription billing platform	5.5k	+57/wk	Java	Apache License 2.0	77
App Welcome to New Expensify: a complete re-imagination of financial collaboration, centered around chat. Help us build the next generation of Expensify by sharing feedback and contributing to the code.	4.9k	+5/wk	TypeScript	MIT License	75

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Our Analysis

Lago9.7k★

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.

turboquant_plus6.8k★

TurboQuant Plus is a quantitative trading platform for building and backtesting trading strategies. Python-based with support for multiple data sources and strategy templates. If you're developing algorithmic trading systems, this provides the framework for strategy development, historical backtesting, and performance analysis. Fully free, no paid tier. The catch: documentation and community are primarily in Chinese. Quantitative trading tools require significant domain expertise in both finance and programming to use effectively. No English documentation means you need Chinese language ability or patience with translation tools. And backtesting performance doesn't guarantee live trading results. The trading platform space has established alternatives like Zipline and Backtrader with English-language communities.

Kill Bill5.5k★

Kill Bill is a full billing and payments platform: subscription management, invoicing, payment processing (pluggable gateways: Stripe, Braintree, Adyen, etc.), tax calculation, revenue recognition, and multi-tenancy. It's been around since 2010 and handles real money in production. Apache 2.0 license. Used by companies processing billions in payments. Self-host for free. The company behind it (Kill Bill Inc.) offers paid support and consulting, but the software is fully open. Small teams with significant billing complexity (usage-based pricing, multi-currency, custom invoicing), should evaluate Kill Bill. If you're just charging $20/mo for a SaaS, Stripe Billing is simpler. The catch: this is enterprise-grade Java software that's complex to deploy and maintain. It runs on a JVM, needs a database, and the documentation assumes billing domain expertise. The learning curve is steep. For simple subscriptions, this is a bulldozer for a garden.

App4.9k★

New Expensify is a ground-up rewrite of the expense management platform, built as a React Native app that runs on iOS, Android, web, and desktop from a single codebase. It handles expense reports, receipt scanning, corporate card management, invoicing, and bill pay. The open source angle is interesting: Expensify publishes their entire client app on GitHub, which is rare for a fintech company. But this is not a tool you self-host. It is the client for Expensify's paid service. Contributing is encouraged but the product only works with Expensify's backend. Alternatives like Fiskl and Budget Zen exist for personal expense tracking, but for corporate expense management the real competitors are SAP Concur, Brex, and Ramp. The catch: open source client, closed source backend. You cannot run your own Expensify. This is transparency, not freedom.