Graduated Neutral Density filters. For many years my GND filters go everywhere my camera goes. Probably my most used filters. Of course, they attract dust like a magnet which is not much fun when you're trying to set up in a hurry.
For a while now, I've been using the GND filter in the Nik software suite. I like this a lot, with all the flexibility and everything.
Does this make glass filters obsolete?