Let me restate your problem: Case (1) When you save from PS normally (ie, using the "save as" command), you images look different depending on where you post them, what browser you use to view them, etc. Case (2) When you save from PS using "save for web" images are much more consistent, even if you post them on different websites, view them in different browsers, etc.
I'm 99% sure that the cause of this is that your working color space (in PS) is set for ProPhoto (aka, ProFoto) or some other wide-gamut space or you are using an output device color profile that wasn't intended to be used as a working space (eg, using a printer profile as your working color space).
When you do a "save as" from (say) a ProFoto-tagged image, the JPG that PS produces is also in the ProFoto color space, whereas when you do a "save for web" on the same image, the resulting JPG is almost always automatically converted into the sRGB color space before it is saved. This is exactly what you want to happen.
The reason the ProFoto JPGs look different in different browsers or different places on the net is because some browsers don't know how to interpret images in the ProFoto or other advanced color spaces, so they simply interpret the file as if it was a sRGB image. This leads to the very characteristic sickly brown skin look in your 2nd photo. The same holds true for the software that runs on-line forums and photo hosting sites.
If you don't want to be bothered by all of this, just be sure that your working space is set to sRGB and that you have set up PS's preferences to warn you if there is a color space mismatch when you are bringing files into PS. If a file triggers this warning, always select the "convert to sRGB" option, NOT the "do nothing" or the "assign to sRGB" options.
Once you do this, you will get maximum compatibility with the widest range of browsers and forum photo uploading software. In addition, you will get the same results whether you do an ordinary "save as" or a "save for web".
HTH,
Tom
