I recently made a SplitMix implentation:
http://hackage.haskell.org/package/splitmix

When generating `Word64`s it's noticeable faster than `tf-random` (and
`mwc-random` and obviously `random). I didn't benchmark `Double`
generation, as `tf-random` own `Random` class doesn't support it [1],
and using `random`'s class would be very unfair.

I haven't announced it (well, mentioned on Twitter), but please give it
a try!

- Oleg