Firstly: https://kaspa.org/

Apparently similar to Bitcoin, a trustless cryptocurrency. A modernized consensus engine allows for a more efficient system with no wasted mining power, more frequently generated blocks and thereby more transactions.

Appears to solve for immediate transactions which may limit the ability for Bitcoin to replace the immediacy of the existing systems used to support fiat currency transactions.

Currently has a $5b market cap, but lacks the network effect of Bitcoin.

I'll be doing more research into this one but wanted to share it here in case others have looked into it already and have views, or to provide the opportunity for others to look into it.