The AT89S52 (note the prefix, it is important) is pin compatible with the original Intel 8051 (Intel does not manufacture microcontrollers anymore, but it is Intel where the 8051 was originally developed more than 30 years ago). There are many manufacturers of 8051-compatibel microcontrollers, and quite a couple of them manufacture devices which are still pin compatible with the original 8051. There are also pin-compatible derivatives with extended features, such as program memory up to 128kBytes of FLASH, internal RAM up to 8kBytes, a wide selection of peripherals, etc. Look at the table at http://www.keil.com/dd/parms/8051.htm - most if not all of the devices which are listed with DIP40 package are pin compatible with the original 8051. (Note that some of the devices on that list are not manufactured anymore).

As far as the AVRs go, Atmel when started with AVRs wanted to make things smooth, that's why they offered the AT90S8515 which is pin compatible with 8051 except the polarity of RESET pin. This chip is already obsolete; it's current replacement is ATMega8515 (still pin compatible except of RESET polarity).

JW