I was reminded of something I frequently do when reading a recent photo magazine - getting a black and white preview while taking pictures. One of the benefits of digital is that you can shoot color and still post process black and white. Having the color file available to you is amazing, since you can do all kinds of color filtering and mixing to arrive at a final black and white image.
Something unique to digital, though, is sort of a 'best of both worlds' feature. If you have a camera that can be set to record black and white (my Canon DSLR does) and also allows a RAW + JPG setting, then you can take photos and get a black and white preview, black and white JPG, while still having the color RAW file to go back to. This gives you both the advantages of having a color 'negative', with the obvious benefit of being able to view the result in black and white immediately. It also serves as a reminder that you intended the file to be black and white when viewing the previews in your organizer.
Many DSLRs allow color filters in addition to contrast that are applied to the JPG. This isn't as sophisticated as what you can do later with channel mixing, but is a lot closer to getting an idea of what the final image could look like.
Photo credits: Brad Calkins.