The Atmel AVR is a family of RISC microcontrollers from Atmel. The AVR architecture was conceived by two students at the Norwegian Institute of Technology (NTH) and further refined and developed at Atmel Norway, the Atmel daughter company founded by the two chip architects.
Unlike the PIC, the AVR instruction set is strictly orthogonal and compatible across models. The AVR was designed at the outset for efficient execution of compiled C code. Thus some instructions, such as 'add immediate', are strangely missing (you have to 'subtract immediate' the complement instead), while others, such as 'compare with carry', are included. Like the PIC, it has a dedicated hobbyist following (exemplified by the AVRFreaks Internet forum), largely due to the existence of cheap/free development tools, for instance a port of the GNU GCC tools.