PIC microcontrollers were perhaps the first that were marketed to the hobbyist and student community, one of the first microcontrollers to be offered in a relatively small package (18 pin DIP) and one of the first to implement flash or eeprom program memory (in the PIC16C84 in 1993)
Architecture: harvard, accumulator based (mostly)
Package sizes: 6, 8, 14, 18, 20, 28, 40, ... 100
Program memory size: 256 words (12bit words = one instruction) to 256k bytes (2 bytes=1 instruction)
Data RAM: 16 to ~3900 bytes (4096 byte address space, shared with peripheral registers.)
Special features: EEPROM, 20mA output drive, several "sub-architectures"
Flash memory based PIC microcontrollers curently range from vanishingly small 6-pin chips in SOT23 packages to 100pin TQFPs. The basic architecture has been extended to chips with 16bit ALUs and integral DSP functionality.
Microchip has a liberal sample policy and chips are widely available from many vendors.