I know this is somewhere on the web but you could look at your System Preferences under Keyboards/Mouse --> Keyboard Shortcuts and check there. This obviously isn't going to list every keyboard shortcut for every application you have but it will give you most of the system ones. As a general rule, Mac apps usually only use the Command (or Apple) key as the modifier key. So, if you stay away from the Command key you (generally) should be OK.

You can also use Service Scrubber to remap keyboard shortcuts in Services.

I haven't found a comprehensive list of unused keyboard shortcuts. Please let us know if you find one.

Here's a link to link to all the system reserved keyboard shortcuts. I can't find anything that lists strictly unused ones.

To add to this pile of info:

all of the keyboard shortcuts in Mail.app (.pdf)

freeware: RUnit, to assign your own shortcuts to terminal commands

"" "" Shortcuts, "" "" for contextual menus

and the Magical Macintosh Key Sequences article

AHA! I think I found it, in the ADC:

in the Human Interface Guidelines section: see Appendix A of this article
(you can download the whole HIG in .pdf, as well)

more ferreted links, this time all related Dashboard Widgets:

1. xCuts (every.single.one. :eek:, and organized heavily to boot, with a tiny footprint)
2. SafeCuts (Safari)
3. CharacterPal (special characters)

Thanks for this information.
Thanks for all the kind words of support!

"a long list of modifier key tricks" @ MacOSXHints