"Nuts.
Loony.
Bats, in the belfry or not
Cuckoo
Mad, as a hatter or not
That last is more Brit than murrican"

Nutty as a fruitcake
Daft as a brush
Away with the fairies
Mad as a box of frogs
Barking
Doolally-tap
Crazy as Joe Cunt's cat

(Sorry about that last one but it ain't half good...)