Updated

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 U.S. 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.

First-time users of the person-to-person cash-transfer service will find a secure link in Square’s email to a site requesting a debit card number. Enter the data and the money will be sent across within a couple of days. Once the debit card link is established, future transactions will take place automatically.

That’s right, you don’t need to set up an account with Square, and even better, there are no charges to think about. All that’s required is a good old-fashioned email address and a debit card with a Visa or MasterCard logo. The current weekly limit for transfers stands at $250, whether separately or as one single amount. You can increase this to $2,500, though to do so you’ll need to provide some additional information.

Not surprisingly, security is a top priority for Square, with a special page on its website dedicated to the subject.

The San Francisco-based team has also launched two apps (iOS, Android) for the service, although they essentially help you compose the email and that’s all.

“Square Cash makes it convenient to send money to anyone, without making them jump through hoops to retrieve it,” Square Cash lead Brian Grassadonia says on the service’s website. “Now it’s easier than ever to split a bill, send a birthday gift, or settle up with a friend, no matter where you are.”

Square Cash’s simplicity and speed could well make it a big hit with those wanting to send small amounts of cash fast or when meeting up to pay is difficult. If you’ve had a chance to try it, let us know how you got on in the comments section below.