There are some issues that are still prevent using their certificates in production¹. For example this one prevents Windows XP clients (not just IE, but anything that uses system X.509 libraries) from connecting: https://github.com/letsencrypt/letsencrypt/issues/1660.

Still there's WoSign (free for everyone) and StartSSL (free for non-commercial use).

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¹) Only where 3-5% of more obscure clients still matter.