Tari is an evolution grin’s mimblewimble chain combined with being merge-mined with monero for increased security. The testnet has been operational for a while, although I don’t follow it closely and haven’t heard anything since they were stress testing it a couple months ago.

https://tarilabs.com/

It's been a while since I looked into Grin. My understanding is that it lacks some privacy features compared to Monero, which is what allows Grin to scale better.

Mimblewimble is about simplicity and elegance which benefit both scalability and privacy. It solves 2 of the 3 major privacy issues (amounts and addresses), while leaving potential to reduce the 3rd (linkability) by making coinjoins trivial (tx aggregation). Monero has much better privacy, but it pays a heavy price in having 20x more chain bloat, and needing to have random access to some info of all spent outputs. It also lacks (scriptless) scripts which makes possible things like payment channels.

Interactive protocol, non-private notional.