Let's Encrypt is a free, automated (using ACME protocol), open CA that signs Domain-Verified certs for free. Signed certs work with all major browsers.
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Lets Encrypt is a non-profit organization sponsored by EFF, Linux Foundation, and other major donors. Removing profit as a motive means their focus is security, stability, and open source, not their bank account.
Certs from LetsEncrypt are free, always, not just as a short-term trial.
It is fairly straightforward to totally automate certificates and renewals such that you never have to worry about the dreaded "certificate expired" error. There are numerous open-source implementations of clients to fit many situations.
Pro Very easy and quick to add dozens of domains and sub-domains.
Con 90 days
Certs currently are only issued for 90 days at a time. The expectation is that users will automate renewal so that this is not an issue.
Con Domain-Verified only
Only class 1 certificates are available (the kind others charge $10 or more for). Lets Encrypt does not do Extended Validation or EV certs, nor does it offer insurance or the other features traditional vendors offer. How much of a con this is depends very much on how much you value those services.