Nepenthe by Amalgamated Signals---Free
https://amalgamatedsignals.com/nepenthe
Nepenthe is a reverb designed to be smooth, transparent, and easy to use.
Nepenthe's algorithm is based on the concept of velvet noise. Rather than building echoes with a network of delay lines, it uses a predetermined set of randomly-generated echoes. This is a very simple design, which works well with modern CPU optimizations, so Nepenthe runs efficiently.
Nepenthe is open source, and its code is annotated with detailed explanations of the design.