Microcontroller after Arduino

[pyrohaz]

Since you've already learnt the Arduino, i'd recommend moving to the Maple which is pretty much an ARM version of the standard Arduino. It really helps to learn the "near" lower level commands with arm processors like accessing GPIO registers. After you've got a feel for the Maple such as the higher speed and increased functionality, I'd really recommend getting an STM32Fx discovery board, they're utterly brilliant and really cheap! After spending some time on the Maple, moving to the discovery boards is pretty easy.