I've got a lot of XMOS hardware. The chips can replace FPGAs and DSPs in a lot of applications, with development being much quicker and cheaper. They are mainly programmed in XC (a superset of C intended for parallel processing), C, C++ and assembler. The languages can be mixed in the same application. Other programming languages are becoming available.
They are basically very fast multicore controllers, with up to eight hardware threads per 400 MIPS core, operating in round-robin fashion. Each thread can run at 50 or 100 MIPS, and can be thought of as a separate processor. The four-core device thus offers up to 32 threads, delivering a total of 1600 MIPS. Threads, cores and chips communicate via very fast communication channels, making it very easy to design parallel processing systems using arbitrary numbers of chips. Peripherals like UARTs, SPI etc. are implemented in software. They are fast enough to handle high-speed (480 MBit/s) USB and 100 MHz Ethernet in software. Single-core, dual-core, and four-core devices are available with 64 I/Os per core. On-chip RAM is 64k per core.
Killer applications include those massive LED displays used at sporting arenas, where FPGAs have been used up to now. They typically use hundreds of XMOS chips, one per display tile. They are also ideal for high-end robotic applications.
Board prices start at about 50 dollars for a prototyping board with a single core device. The JTAG interface needed for programming and debugging applications is another 50 dollars. Development software is free. Support is good, via the XMOS web site and a users forum. They are getting popular with hobbyists.
A new $7 XS1-L01A-TQ48 device is now in production. They are listed on Digi-Key.