ZPU is a very interesting soft core processor designed to take as little FPGA resources as possible, leaving you a lot of space for peripherals and hardware accelerators.  It’s a modern stack based 32 bit CPU, the implementation is independent of the FPGA model and it can also be used in other technologies (e.g. anti-fuse, ASIC).

For example, on a Xilinx FPGA, ZPU takes 442 LUT @ 95 MHz with 32 bit datapath. It also comes with GCC toolchain (GDB, libstdc++) which the software development an easy task. It supports some realtime operating systems, like eCos and FreeRTOS.

What makes ZPU really attractive even for commercial use is it’s license: the RTL is released under a FreeBSD license which makes it compatible with most of the open source licenses around, but also allows it to be used in proprietary products.

The software and the documentation are released under a GPL license, which also a very friendly one.

ZPU has even been used by the big guys, like National Semiconductor which used it in a development kit.

You can read more on their official page at http://opensource.zylin.com/zpu.htm