The Atmel AVR is a family of RISC microcontrollers from Atmel. Unlike the PIC, the AVR instruction set is compatible across models. The AVR was designed 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) and others, such as compare with carry, are included.
Like the PIC, it has a dedicated hobbyist following (exemplified by the "AVRFreaks" forum), largely due to the existence of cheap/free development tools, for instance a port of the GNU tools.