PicoRV32[0] targets some extremely low spec FPGAs. I have it running on ridiculously small and cheap FPGA[1] that has 8k LUTs and cost me £41 including tax and next day delivery.
However I repeat what I said above: hobbyists are free to design their own instruction sets, and good on them!
The ZipCPU and several peripherals can fit within an iCE40 8k using only about 5k LUTs--and that's with the multiply unit, the divide unit, the pipelined fetch unit, and a debugging bus infrastructure. While that isn't the full pipelined implementation neither do the caches fit, it is sufficient to be competitive with the picoRV.
The picorv32 is a nice little processor, but it's surprisingly inefficient in its use of FPGA resources compared to the performance it offers (which is very low.) For small FPGAs, it's far from being the best solution.
That said: there are a bunch of SOC construction tools for it, so that's definitely a redeeming factor.
