The SEAD-3 is an evaluation platform made by MIPS Technologies. It is meant to support the development of SOCs so it features logic analyzer connections, a JTAG port and other features to support this specific purpose. Other than that it provides a very barebone environment, two 8250-class UARTs provide the only connectivity to the outside world.
The SEAD-3 is fully supported by Linux 2.6 and current Linux 3.x kernels. The main Git tree hosted on linux-mips.org is geared towards bleeding-edge development. There is a separate tree that is geared towards absolute stability on SEAD-3 as well as the Malta platform. It is actively maintained by MIPS Technologies, Inc.
The SEAD-3 board does not currently have any full-system emulation support.