Return the flipped version of a number
Haskell - 43 31

43 characters without anything fancy.
f 0=0
f 8=8
f 2=5
f 5=2
f 6=9
f 9=6
f _= -1

Got it down to 31 characters by making it a partial function.
f=([0,-1,5,-1,-1,2,9,-1,8,6]!!)