Philips and Atmel make a range of 8051-family compatible Flash memory micros. Philips make the P89C series (89C 52 / 54 / 535 / 536 / 538), Atmel the AT89C series (89C 1051 / 1052). Atmel also make the AT89S series which are essentially the same as the AT89C series but can be serially programmed.
Atmel designed their own micro in flash technology for integration into ASICs. It happens to be very fast and efficient as well. It has a pinout very similar to the 8051 but not identical. This minimises work if changing designs from 8051 family. It comes in 40 and 20 pin versions, but there are no equivalents to the 28-pin PIC micros.
I'm fairly new to the Atmel AVR micro. I like the fact that it has Flash because I don't have to buy UV erasers and expensive quartz windowed packages, or wait 30 mins for erasure. I don't like the fact they have no I2C interface. I hear a few murmurs of discontent about the Atmel chips but nothing concrete.
Myke Predko's website has lots of stuff for the AVR, PIC, and 8051.
Larry Barello's website
says Atmel's AVR assembler is rubbish. From my quick first
experiments, I recall that it could not do things I expected such
as nested macros.
Larry Barello's
AVR tools page has a link to IAR's assembler
(self-extracting 1.3Mbyte file aa90.exe) which he recommends
using instead. I have downloaded this but have not tried it yet.
This is a self-extracting zip file containing aa90.arj and an
installation program.
Larry Barello's
AVRX page has a link to a Real Time
Operating System kernel called AVRX which he wrote.
BASIC-52 is a very well known BASIC for the 8052, widely available in binary or source code. This can be very useful for debugging 8052 products. It can run in external ROM, although the EPROM-programming commands won't work as they need to run in on-chip ROM. With Atmel's programmable 89C1052, all commands including the EPROM programming will work.
BASIC compilers for the AVR do exist but I don't know if an open-source interpreter exists..
Sourceforge project winavr is a suite of software tools including the Gnu C for the AVR.