Trading: Good paypal alternative?

I've noticed how much trading goes on here. What are some good alternatives to paypal in the US?

What kind of alternative? Why not just use PayPal? I think it's far and away the most used in the US.

Payza (Formerly AlertPay) is decent for US users.. Liberty Reserve is used widely across the world.

PayPal is the biggest, most popular, for the US and rest of the world for the most part.