Some of the best free OTP (One-Time Password) tools and APIs

1.Twilio: Twilio offers a free tier that includes some free OTP
messages each month. It's a popular choice due to its
extensive documentation and support for multiple programming
languages.
- Twilio Pricing
- Twilio Documentation

1. MessageBird: MessageBird also provides a free tier with some free credits to start with. It's known for its ease of use and reliable service.
   • MessageBird Pricing
   • MessageBird Documentation
2. Authy: Authy, a service by Twilio, offers a comprehensive OTP solution with a free tier for developers.
   • Authy Pricing
   • Authy Documentation
3. Firebase Authentication: Firebase by Google provides a free tier that includes phone authentication via OTP.
   • Firebase Pricing
   • Firebase Documentation
4. Vonage (formerly Nexmo): Vonage offers a free tier that includes some free credits to get started with OTP services.
   • Vonage Pricing
   • Vonage Documentation
5. Sendinblue: Sendinblue provides a free tier with a limited number of OTP messages. It's a good option for small-scale projects.
   • Sendinblue Pricing
   • Sendinblue Documentation
6. OTPlib: OTPlib is a Python library that can generate OTPs, but you will need an SMS gateway to send the OTPs to users.
   • OTPlib GitHub
   • OTPlib Documentation

When choosing an OTP tool or API, consider factors such as the number of free messages, ease of integration, and support for your development platform.