Some thoughts:

An image on a Mac looks lighter than the same image on a PC.

LCD screens have a very different gamma than a CRT, and most people with LCDs do not have any special gamma software to compensate. Images made to look good on an LCD often look too dark on a CRT.

I created the following alignment page several years ago specifically for CRTs on a PC. It is not accurate for LCDs or Macs, though may get you closer than nothing on those too. Be aware that to even see the dark gray boxes within the black box at the bottom of that page you will have to stare at the box for several seconds. Even the lightest of those 5 boxes should seem black when you first glance at it.

http://www.djdesign.com/test/monitor/