Earlier this year I was about to buy a graduated neutral density filter. These filters are mostly being used to darken the sky in landscape shots. I did a lot of research and in the end wanted at least two filters in the expensive glass 100x100mm version which I would then simply hold and move in front of my lens to have some flexibility.
In the end I didn’t buy anything for three simple reasons: (1) About 100€ for two good filters is just too much for those rare scenes I would really use them (at least in my eyes), (2) It’s pretty inconvenient to carry around when traveling light and (3) I would need an additional (and expensive) filter holder for long exposures.
My solution (to at least some degree) is to use the Graduated Filter tool in Lightroom. I used this technique a lot on my recent B&W Photos. The good thing is, that you have full control about the intensity, the height, the strength and the position of the effect.
I don’t know if that’s common practice, but at least I wasn’t aware of it and didn’t do it before. Matt Kloskowski at lightroomkillertips.com did a great video about it: link.
The downside is that this doesn’t help in extreme situations. Most people say you have about 1 f-stop backup if you’re shooting RAW. You can go lower, but you will loose quality. So in very extreme situations you will still need a dedicated Grad ND, or you can do HDR or exposure fusing. But for me this technique works well enough.
Below I attached a before and after (-2 Stops) screenshot of the Graduated Filter effect in Lightroom.
Hi, my name is Chris. I am a wannabe photog, traveler & geek that lives in Hesse, Germany. 
follow me on Twitter
A travel, photography & nerd blog