You need to enter your bitcoin address to get inputs and create a transaction, and you need to enter your private key to sign the transaction (coinb.in runs client-side in your browser with javascript, so that the coinbin server never sees your private key, but you can download coinb.in source and sign the tx offline).
I also used Guarda web wallet (https://guarda.co) for small altcoin transactions, but I dont know how secure it is for large amounts.