Allthough others here might recommend other micros I would suggest the AT89S52:

http://www.atmel.com/dyn/produ...rt_id=1918

This part is ISP programmable by the help of "ISP programming cable":

http://www.atmel.com/dyn/produ...ol_id=2877

Don't build this thingy by yourself, but buy it! It uses special parts which aren't easily available.

These two links show the ISP feature in a typical application:

http://www.8052.com/forum/read/109412

http://www.8052.com/forum/read.phtml?id=122673

To write your code in assembler you can use this editor:

http://home.arcor.de/jensaltmann/JFE/jfe_eng.htm

And here you will find a nice assembler which transforms your code into a hex-file needed by the ATMEL's ISP programming software:

http://plit.de/asem-51/

Kai