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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Pros
Pro Non-Profit
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.
Pro Free
Certs from LetsEncrypt are free, always, not just as a short-term trial.
Pro Automated
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.
Cons
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.