Seen this on slashdot the other day:

40 linux friendly single board computers

Mostly ARM based but there's also MIPS and a few x86s in there as well. Alas not a single PPC.

There are some they've missed, Qualcomm and Nvidia boards aren't mentioned and neither is the firefly.

I don't think there's any 64 bit boards yet but Allwinner already showed one.