lowRISC is not the only open source processor project based on RISC-V instructions, as researchers at ETH Zurich university and the University of Bologna have developed PULPino open-source processor based on RISC-V instructions set, optimized for low power consumption, and targeting wearables and the IoT applications.
PULPino is a single core processor derived from the PULP project (Parallel Ultra-Low-Power Platform) featuring a quad core RISC-V SoC with new RI5CY Signal Processing ISA extensions designed by the universities.
The core has an IPC (instructions per cycle) close to 1, full support for the base integer instruction set (RV32I), compressed instructions (RV32C) and partial support for the multiplication instruction set extension (RV32M). PULPino also features peripherals such as I2S, I2C, SPI and UART.
PULPino has already been taped out as an ASIC in UMC 65nm at the beginning of the year, but the RTL code be run on Xilinx Zynq-7010 powered Zedboard, and all source files, test programs, and tools have been released in github under the Solderpad hardware license derived from the Apache 2.0 software license meaning you can basically do what you want with the design.
An implementation of FreeRTOS is said to be available for PULPino and PULP processor, but I could not find it. They’ve also compared RI5CY core to ARM Cortex-M4 to show a similar area and power consumption using 65nm process.
You can find more details about PULPino and PULP projects on Pulp Platform website, and PULP page on ETH Zurich university website. lowRISC.org also mentions there are three proposed projects for PULPinfo as part of Google Summer of Code: porting CMSIS-DSP to PULPino, Doom on PULPino, and porting the Arduino libraries to PULPino.