I recently tweaked around a little with custom tone curves to enhance the JPEG output of my D80. I started with a very high contrast curve and reduced it over the course of a few shootings comparing to the Auto mode. It is still very high contrast and you can see how it looks like in the following picture.
I had little experience with custom curves in the past and I tried some highlight saving curves which I couldn't get right and which required quite a lot of post processing. I came to a point where I asked myself if in such cases it wasn't better to shoot RAW right away and try something different with the custom tone curves. With the current curve I'm quite happy, it has rich colours a nice black level and increased contrast. I don't do a lot of post processing and this does pictures more in the way I want them.
Here are a few examples
All are taken with the Sigma 30mm f/1.4 that I borrowed from a friend for a week. Judging from a few photos it's an excellent lens for a reasonable price and finally in the range of true normal lens (45mm equivalent) than my other fast prime (50mm f/1.8) which I always felt too narrow (75mm equivalent). I think the ideal focal length for me would be 35mm equivalent though so that would be 24mm on my D80. Unfortunately the Nikon 24mm f/2.8 is slow for being a prime and all but praised and the Nikon 24-70 f/2.8 is a beast I don't like to afford. I think I'll go after another Sigma and get their newly announced update to the 24-70 f/2.8 when it's released and when the price is okay and maybe this Sigma 30mm. Nikon is taking way too much time to update their wideangle prime lenses.
UPDATE: I tweaked the curve slightly. And you can download it here: www.ipernity.com/doc/19240/3297275