Most people use PayPal to purchase stuff on eBay, but you can also use it for any kind of payment. And if you need to send money to a relative or friend, you can do it with dangerous ease from your BlackBerry with the newly-released and prosaically-named Send Money app.
As Nick at Gizmodo humorously points out, you could easily exploit this application to score money from unsuspecting rubes. For all that, it's a handy way to perform quick funds transfers on the go. Send Money is a free application, available from the recently liberated BlackBerry App World.