Hidden boards on GameFAQs that cannot be found without going through random numbers or obtaining a link elsewhere. They are the following (board numbers shown):

Brilliant - -1
Nine - 9
Board 250 - 250
Eggbeater - 486
Spork - 542
Spatula - 622
Semprini - 709
Jenny - 8675309
Lame - 20040401
Rnd() - 123456789
Pie - 314159265
Toaster Oven - 909090909

Currently, Toaster Oven is the Special Board with the most topics, with Spork in second.