Your last must-have filter is the graduated neutral density filter. This isn't as versatile as the other two filters, and really, you only need it if you're planning on getting out and about to do some serious work with both sky and land in the same shot—a given for most photographers in the great outdoors. Think sunsets over the sea, dawn over the mountains, the like.
For such shots, it's incredibly difficult to get both the heavens and Earth exposed correctly. In fact, it's often impossible to do this on the same shot... unless you have a graduated neutral density filter handy.
Basically, graduated neutral density filters are a piece of filter that go from dark to light with some sort of gradient. This gradient can be either abrupt, or over a wide space. They may also go from clear to varying levels of darkness, from near pitch black to a subtle pale grey. Between these two variables, you've got a lot to choose from.
Now, unless you want to get a whole set (expensive), you'll be somewhat limited with the one you'd be purchasing. I'd play around and see what sort of composition you prefer for such shots, and thus what sort of graduated ND filter would be best for you.