"Single board computers" encompasses a pretty wide range of hardware, a lot of which has a lot more power than an Arduino. The next immediate step up would be a

"Single board computers" encompasses a pretty wide range of hardware, a lot of which has a lot more power than an Arduino. The next immediate step up would be a 32-bit microcontroller board like Arduino Due. Above that would be the most basic of Linux-capable boards, such as say the Rascal Micro. Then there are boards based on smartphone SOCs like Beagleboard and Raspberry Pi. Beyond that there are definitely x86 single board computers although the market starts to get more specialized. All of these generally provide access to GPIOs (as well as embedded-friendly serial IO like I2C, SPI, TTL-level UART). Or if you really need something like a modern desktop except with really high bandwidth access to GPIOs, consider something like a PXI chassis. Or find a PCIe card that provides GPIOs - I've never had to shop for such a thing but I bet there are suppliers targeting industrial markets.