Here are some of the best free API documentation options for a startup on a tight budget, grouped by what they’re best at:

1. Swagger / OpenAPI with Swagger UI

Best for: Standard, widely supported API docs
Why it’s good:

• Free and open source
• Excellent for REST APIs
• Interactive “try it out” docs
• Strong ecosystem and tooling

Tradeoffs:

• Requires maintaining an OpenAPI spec
• Styling/customization can take extra work

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2. Redoc / Redocly Open Source

Best for: Clean, polished, developer-friendly docs
Why it’s good:

• Beautiful default layout
• Great readability for API reference docs
• Works well with OpenAPI specs
• Open-source option is free

Tradeoffs:

• Primarily documentation, not a full API portal
• Advanced features may require paid Redocly plans

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3. Postman

Best for: Teams already using Postman collections
Why it’s good:

• Easy to generate docs from collections
• Good for internal/external sharing
• Built-in testing + documentation workflow
• Free tier is useful for small teams

Tradeoffs:

• Documentation is tied to Postman workflows
• Not as ideal as OpenAPI-first docs for large public docs

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4. Stoplight

Best for: API design + docs in one place
Why it’s good:

• Great for designing and documenting APIs
• Visual editor is helpful for teams
• Can publish attractive docs
• Free tier can work for startups

Tradeoffs:

• Free plan has limits
• More SaaS-dependent than pure open-source tools

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5. GitBook

Best for: Lightweight, general product/docs sites with API reference sections
Why it’s good:

• Easy to write and publish docs
• Nice-looking hosted docs
• Good for combining API docs with guides/tutorials
• Free tier available

Tradeoffs:

• Not API-spec-native in the same way Swagger/OpenAPI tools are
• Advanced collaboration/features may cost

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6. Docusaurus

Best for: Full docs sites with strong customization
Why it’s good:

• Open source and free
• Great if you want docs + tutorials + changelogs
• Works well with MDX/Markdown
• Flexible and developer-friendly

Tradeoffs:

• More setup/maintenance than hosted tools
• Best if you have someone technical to manage it

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7. ReadMe

Best for: Polished developer portals
Why it’s good:

• Excellent UX for API docs
• Interactive, modern, and developer-friendly
• Good onboarding and API reference features

Tradeoffs:

• Free offering is limited; most startup-scale use will likely need a paid plan
• Better if documentation is a core product asset

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Best choices by startup stage

If you want the lowest-cost, best-standard option:

• OpenAPI + Swagger UI
• Redoc

If your team already uses Postman:

• Postman documentation

If you want a nice docs site with minimal effort:

• GitBook

If you want full control and can handle setup:

• Docusaurus

If you want docs + API design workflows:

• Stoplight

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My practical recommendation

For most startups with limited budget:

1. Use OpenAPI as the source of truth
2. Publish with Swagger UI or Redoc
3. If you also need guides/tutorials, add Docusaurus or GitBook

This gives you:

• free or very low-cost documentation
• portability
• fewer vendor lock-in issues
• a path to scale later

If you want, I can also give you:

• a comparison table
• the best option by use case
• or a recommended stack for a small startup team