Optimizing photos for web display (1 Viewer)

I have a calibrated monitor. I can achieve quite accurate prints using the printer profile to review my pictures on my display.

However, what I noticed is that some of the photos I post to online (e.g. BirdForum) look not so great when viewed on other non-calibrated monitors. A particular problem is Mac displays which appear to have a factory gamma value of 1.8. I tried changing the gamma value to 2.2 on my a mac I have access to and my pictures looked much better, though not as good as on my calibrated monitor.

I noticed that pictures on some popular websites (e.g. CNN, Yahoo, etc.) look good on various PC and Mac monitors. Looking at the histograms, they appeared to be more compressed than a typical fully exposed image from a camera. Is there a photoshop utility or a third party tool to "compress" images such that they look decent on most monitors out there?

have you eliminated colour space? Ideally, images intended for web use should be in sRGB, anything else will result in colour shifts.