Another option I use is a linear polariser stacked on a circular polariser. It's gives a poor mans
Singh-Ray and produces a variable ND (not to be confused with a graduated ND)
With this I can get up to about 2-5 seconds exposure on very bright midday sunshine. You can do much longer, i.e. 10 - 20s, but a very strong blue colour cast is evident when you push it too far.