And after Cmd-Tab, Cmd-~ takes you backwards in the list of apps. So if you Cmd-Tab too many times, Cmd-~ takes you back instead of having to cycle through.
Likewise, in Safari Cmd-} (aka Cmd-Shift-]) and Cmd-{ take you forward and backward through the tabs. This also works in Terminal.
Cmd-L puts the focus on the location bar in Safari if you want to type in a URL (or tab over to the search)
Quicksilver is cool, but I just use Spotlight for launching apps. Cmd-Space turns on Spotlight.
If you use TextMate, I highly recommend the TextMate book from Prag Press.
Those are some of my favorites.
Being new to the Mac, I'd be happy to hear more shortcuts!
Just enable on Triggers, I set Shift-Ctrl-Cmd-V, and then I can use 1-9 to paste any of the 9 most recent clipboard items. I can use the mouse to select any of the most recent 250 clipboard items.
Also, when Cmd-Tab'ing, you can hold down Cmd and hit q to quit an application. It's a quick method for closing a lot of applications.
Cmd-Tab switches between applications (as I'm sure everyone knows), but it's less useful on OS X than Ctrl-Tab is on Windows or Linux because it doesn't switch between individual windows in those applications.
Cmd-Tilde (~) was the missing link for me. It switches between windows in the current application. Those two are probably the most important keyboard shortcuts I know on OS X.
Plus, another endorsement for Quicksilver. That's always the first application I install, and I now find it really frustrating to use any computer that doesn't have it installed.
And maybe you should set your Dock to auto-hide, so you're not so tempted to move the mouse over there to monkey with things.