I had developed using professional IC design tools (specifically Modelsim and Xilinx) for the processor. However, free tools are available like http://iverilog.icarus.com/

The actual processor was implemented on a Xilinx FPGA, and it was about 2K lines of HDL code, carrying about 25 application-specific instructions which I had designed also.

For the compiler, disassembler, IDE, emulator, I created in C# and .NET.

While I did not use these, there are also ASIPs: https://en.m.wikipedia.org/wiki/Application-specific_instruc...

Not tools per se, but check out the Bitwise education project, where Per Vognsen is coding an entire hardware and software stack from scratch, live on stream. https://github.com/pervognsen/bitwise