Flora-2

Flora-2 is a free and open source declarative logic programming language. ErgoAI is a commercial version of the same language offered by Coherent Knowledge.

Both are built on top of the XSB Prolog variant that uses tabling. They feature higher-order logic, where the names of predicates can be replaced with variables.

They both also feature a “frame syntax” that allows for specifying a data structure and an infix-style syntax, which is different from a lot of Prolog languages.

The differences between ErgoAI and Flora-2 are considerable. In addition to commercial support and deployment features, additional libraries and integrations, ErgoAI adds the ability to generate explanations for conclusions, natural language generation based on ErgoAI encodings. Flora-2 (last I checked) was also missing a function for calculating the difference between two dates.