Personally, I'm a big fan of the Singh-Ray Vari-n-Duo. It's a variable ND filter stacked with a circular polarizer, too. I have the standard mount, but Singh-Ray also makes a thin mount. Even a thin mount version of the filter wouldn't be thin enough to prevent vignetting on really wide angle lenses, though. Below are some photos I've taken with the Vari-N-Duo--you should be able to right click and choose "Original" to see the full resolution picture (to check for quality using the filter).


Burgess Falls by budrowilson, on Flickr


Burgess Fall Tributary by budrowilson, on Flickr


Samantha as Aphrodite by budrowilson, on Flickr

If you don't want to spend that much for the Singh-Ray, B&W makes some great ND filters, standard and thin mounts.