Excellent question OP! 

I would also like to know, for BTC and BCH, is there anything like that?

For Ether and Monero there is but what about other cryptos? An account-less web interface that allows importing a private key and creating transactions?

Electrum is great but it needs a system that can run it locally, while MEW can be used from anything that has a browser, even a public computer in an Internet cafe.

Pretty useful in certain situations I am sure...