I use a B+W110ND which is a 10 stop neutral density filter. It's a bit of a hassle because you have to focus and set up your shot before you put the filter on, but it's no big deal really. I pretty much got this filter so I could take super long exposures during the day.
Here are some shots I took with it. All of these were also stacked with a CP filter:
Photos using a 10 Stop ND Filter