TD-linux develops codec libraries.

I develop crypto code. Testing on BE is nice, not just because of endian neutral code but testing on something 'weird' can turn rare faults into common ones.

Beyond endianness I've found interesting bugs by running code on HP-PA (stack grows in the reverse direction) and on Itanium (misalignment causes a trap to the kernel)...

I wish there were a qemu supported fake architecture that made every optional decision differently (or at least every one that works with most code...).

Mainly for networking or disk storage, and in general for anything involving storing integer data into raw packets that will reach different CPU architectures (e.g. load or store an integer from/to a memory buffer into/from a CPU register).