Which microcontroller trainer did you have in mind?
I have one for an Intel 8008, but it's on the back of a manilla envelope. The software is on paper tape and it can be read using an ASR-33 teletype with a current loop interface at 110 baud. The boot ROM is a 1702, and it has a whopping 1K of RAM that takes only eight chips.
I also have a KIM-I which is a bit newer and has a bit more ROM and RAM. I think this schematic is available on the internet. Source code for the monitor ROM is on the internet as well. There are numerous freeware assemblers for the 6502 processor.
I think schematics for every microchip board made are available as application notes from the microchip website.
Atmel, Philips, and Maxim-Dallas all publish schematics of their development boards and part programmers as application notes. Try their sites out, I think you'll be impressed.