Launched earlier this year as an invite-only service allowing users to send money via email and without charge, Square’s Square Cash on Tuesday opened to all, or at least to those aged 18 or over with a US bank account. Co-founded by Jim McKelvey and Twitter co-creator Jack Dorsey 2010, Square first came to public attention with the launch of its card reader for mobile payments. Its latest venture, Square Cash, looks to be one of the simplest methods yet for making cash transfers online. Say you want to send some cash to a relative or friend as a gift or to pay off a small debt. Simply enter their email address in the To: box, cc it to cash@square.com, and then enter the $ amount in the email’s subject line. If you want to add a message, and you probably will, tap it out in the main section in the usual way. The recipient will receive your email together with another from Square asking if you want to accept the payment.