I consider Spencer Owen's RC2014 (http://rc2014.co.uk) the new S100 of the ancient computing era.
Spencer started off by designing a CPU and a family of modules around the ancient Zilog Z80 CPU. This is actually very similar to the original S100 bus design that was a derivative of the 8080 bus.
Since I'm a modder at heart, I've gone ahead and modified some of the Z80 software that originally came with the RC2014.
I have also developed a number of RC2014-compatible plug-in boards. These include a variety of memory and serial (UART) boards and more importantly, boards that allow the use of other CPUs like the Intel 8085 and 6502 with the RC2014.
This repo holds my mods/additions and general contributions to the RC2014 community.
Software
The source folder contains source code of projects related to the RC2014.
The 6502 sub-folder contains all the 6502-related source code including the Monitor/Debugger and Enhanced BASIC.
The Z80_8080 sub-folder contains all source code for Z80 and 8080 software including the Monitor/Debugger, MS BASIC, IMSAI 8K BASIC (for 8080) and various BIOS.
The original code that shipped with the RC2014 is at searle.hostei.com/grant
The primary RC2014 Github repo is at: https://github.com/RC2014Z80/RC2014
ROM
ROM images you can burn into EPROM/EEPROM for use with the RC2014 can be found in the ROM folder.
Not all combinations of CPU, serial, Monitor/Debugger, BASIC and ROM image sizes are available. When in doubt you can ask questions on the RC2014 group or http://forum.6502.org/.
RC2014 Add-on Modules
The eagle folder contain a number of designs for add-on boards for the RC2014. The designs are in the form of Eagle files which can be used to order PCBs from manufacturers that can take .brd files. Or you can download the free version of Eagle and generate the necessary gerber files for your PCB vendor of choice.
Do also watch out for the OSH Park logos. Those are links to projects that I have shared on OSH Park, and you can order the boards directly from them. And no, I don't make a commission etc from your OSH Park orders.
For a full list of my shared projects, check out this link.
I have put some popular boards up for sale on Tindie.
Lastly, check out the documentation in the docs folder.