Out Testnut mint for developing is

https tesnut dot cashu dot space

It produces an infinite amount of ecash that's - to quote a popular Cashu Dev - backed by 🐴💩

There's also a mint on mutiny net

https mutinynet-cashu dot thesimplekid dot com

This one is backed by nostr:npub1u8lnhlw5usp3t9vmpz60ejpyt649z33hu82wc2hpv6m5xdqmuxhs46turz