Wow! So many things to consider...
Shooting in RAW.
DPP preferences: Working space sRGB. Histogram display: after adjustment (but I haven't made any). NR 0. I don't see any other prefs that look like they would be relevant. Adjustment>work color space is sRGB.
In-camera picture style is neutral>0,0,0,0. Color space sRGB.
I try to keep everything set to neutral so I get as "raw" data as possible.
OK, well, since you guys made me look at all this stuff, I've discovered something: In DPP, the Tone Curve Mode was set to RGB instead of luminance. When I set it to luminance, it pretty much looks like the camera histogram. (And still really doesn't look like the ones in the other software.)
Thanks a lot for all the responses, that's what I needed.
But I don't know what to think about this:
The histogram on the RAW tab is a different "critter", since it apparently portrays RAW data after WB is applied but before the Picture Style adjustments.
The histogram on the RAW tab really doesn't look anything like the one under the RGB tab and my picture style is neutral>0,0,0,0. Would there still be "picture style adjustments" made that would make the histogram look totally different?