The instances are:

Num a => instance Monoid (Sum a)

Num a => instance Monoid (Product a)

Float and Double are instances of the Num typeclass, however, floating point addition and multiplication are not associative. Here's an example:

> (Sum 1234.567 `mappend` Sum 45.67834) `mappend` Sum 0.0004

Sum {getSum = 1280.2457399999998}