Have you checked out Quartz Composer? Its installed with the XCode developers tools (you should have got that on a CD, if not its a free download from Apple). Try it out, you can make your own 3D animated screensavers with movies, images, live-video and effects. Its very popular with VJs or just for making your own screen-savers.
And take some time to muck around with Automator. If there's any task you regularly do that requires a few steps to be carried out, Automator is the place to start. When I had to send a particular report file to the same person every day for a couple of months, Automator helped me there: I whipped up an Automator workflow to compose and send the email whenever I dropped the file onto it. That 3 minutes of fiddling saved an hour of my time over the next couple of months.
Some cool stuff is the Bluetooth connectivity in Address Book. You can read or type SMS messages, get caller ID on the mac's display, and dial numbers. While you're on that track, try out Salling Clicker or Romeo to use your Bluetooth phone as a remote control. Neat.
Another thing that I love about my Mac is that I can print from any application directly to a PDF file, or even directly to a new mail message. Check out that little button on the Print dialog in any program. Finished that spreadsheet or report and ready to send it to your boss? Just go to File -> Print and choose "Mail as PDF" and take the rest of the day off.
Have you also noticed that as well as the traditional copy & paste method of working with text and pictures, you can also select text and drag it around to different programs? When I am doing research on something I'm unfamiliar with, I like to grab snippets of text that helps explain things and dump them all into one document. If you're working in the Terminal you can drag in a file to get its full pathname typed-in automatically. Every day or two, I drag photos straight from iPhoto to FlickrUploadr. Drag and Drop is better on a Mac.
Theres lots more to be discovered: I couldn't get by without the unit & currency conversion widget in Dashboard. RSS feeds in Safari make it easy to navigate and browse your favourite news sites. I'm sure you'll find some cool stuff in there.
