* CloudFlare doesn't give you a certificate. It terminates your TLS connection at their endpoint with a certificate they own the private key to.
* StartSSL free tier is for non-commerical use only.
* Haven't used WoSign so no idea what it is, but clicking on the link took was so laggy it didn't give me much confidence. (I personally wouldn't work with a company in China jurisdiction when it comes to security products.)
Let's Encrypt is a big step forward in having a legitimate, actual, user-friendly, free TLS certificate issuer without restrictions for usage scenarios.