Free SSL Certificate

Let’s Encrypt, and StartSSL will gives you SSL Domain Validation ( Known as Class 1 Validation ) certificates for free

Domain Validation Type

is the lowest level of validation. The certificate authority simply verifies that the organization has control over the domain in question. Often, this is done via email, by uploading a file supplied by the CA to the domain, or by making changes to a DNS record. If the organization demonstrates they control the domain, they will be issued a certificate.

In addition to being the lowest level of validation, domain validation is also the least expensive. It’s easy to automate, and because no humans are involved, there are few costs.

Certificate authorities like Let’s Encrypt are able to offer free domain-validated SSL certificates because the whole process is automatic.

Note
StartSSL won’t allow multiple domains OR wildcards, so you’re pretty much stuck with yourdomain.com or www.yourdomain.com.

For how long are they valid?

StartSSL Provide Free Certificate for 1 year Validation Validity, and 2 years Certificate Validity

Let’s Encrypt certificates are valid for 90 days.
There is no way to adjust this, there are no exceptions. and they recommend to automatically renewing your certificates every 60 days.

Why Free?

Why StartSSL are Class 1 certificates free?
The philosophy of StartCom is guided by the principle that our services are charged according to the effort we have to invest. Since Class 1 certificates are domain and/or email validated only and the process is performed mostly by electronic and automatic means, StartCom doesn’t apply any fees for this type of certification. StartCom started the certification authority a few years ago with the goal to provide free digital certification and adopted a unique business model previously unknown in this industry.

What does it cost to use Let’s Encrypt? Is it really free?
Let’s Encrypt do not charge a fee for there certificates.
Let’s Encrypt is a nonprofit, our mission is to create a more secure and privacy-respecting Web by promoting the widespread adoption of HTTPS. Our services are free and easy to use so that every website can deploy HTTPS.

Let’s Encrypt How to?

Start with StartSSL, How to?

References

1. What Do SSL Validation Levels Mean?.
2. Let’s Encrypt FAQ.
3. StartSSL FAQ.