Ripple has open - sourced a bitcoin plug - in, making the biggest cryptocurrency interoperable with other ledgers. (coindesk.com)
Content Hash - Addressable Storage Market (C# ASM)- extending the «identity», «transaction», and «content» chain thinking that emerged from InterChainZ into an indexable archiving system both as a ledger itself, but also supporting other ledgers; (zyen.com)
Note that Coco is a framework, not a ledger; in fact it uses other ledgers. (techradar.com)