Screenshots - When you take a screenshot using any of the keyboard shortcuts such as Command + Shift + 4, the image no longer appears on your desktop as "Picture 1.png" etc. They now appear on your desktop named as "Screen shot 2008-08-24 3:21pm.png"

Connecting your Mac to a Windows Domain/Active Directory Support - The Open Directory Utility is now gone from the Utilities folder. You can find its functionality now build-in to System Preferences -> Accounts -> Login Options. On right-side of this pane, at the bottom, click the 'Edit' button next to the 'Network Account Server:' line.

Airport - When the Airport card is looking for a WiFi network to connect to it now displays an animation over the Airport icon on the menubar. Instead of staying greyed out (previous behavior), one by one the curved lines of the illustrated radio 'waves' illuminate going upward. A nice visual touch to show your Mac is searching for a network. Before you were only able to determine this by clicking the icon.

Expose - When looking at all windows that are open, Expose now puts the title of the window under the window. When you go to select a window, that window will have a blue boarder. In the case of web browser winodws, the website currently loaded within that window is displayed in addition to the application title.

Dock Expose - Allows you to see small versions of all the open windows in any running program by clicking and holding down on its icon in the Dock at the bottom of the screen. This works great when moving a file from one finder window to another, or perhaps, a new mail message without having to first arrange both windows side-by-side on your screen.

System-wide Text Replacement - Look in the “Language And Text” panel of System Preferences and click on the “Text” tab to set it up and define substitutions. Type (tm) anywhere and the OS will automatically change it to a proper typographer’s symbol. Type “=STANDARDBLAH=” and it’ll be replaced with a whole paragraph making up some excuse why you don't have time to write a longer email because you're so busy with work, etc.

Larger/Interactive Icons in Finder - The addition of a Size slider at the bottom of an Icon View window in Finder allows you to scale icon sizes up to 512 x 512 pixels. These icons resemble large previews of the files' actual contents. For instance, in a small .txt document allowed me to read the small paragraph of text contained within...from the icon! If it's an image, you see the image. If it is a movie or audio file, you can there is a small play button which allows you to play it. If its a document such as a powerpoint/keynote presentation, PDF, or word/pages document, you can press small back/forward buttons to scroll through the pages

Clock/Time Zone - Snow Leopard will use your WiFi location & other hotspots it sees around you to attempt to determine your location, and thus your timezone. When traveling to other timezones, it will reset your timezone accordingly based on your location.

Apple Applications

Google Calendar support built-in to iCal - Open iCal. Go to the iCal's preferences pane. Go to accounts. Click the + symbol near the bottom. Select Account Type as Google and fill in your google credentials. By default, it will sync your main Google Calendar cal, however, if like myself you have multiple personal calendars and subscribe to another person's Google Calendars (my fiance's) you can go to the 'Dependencies' tab on the right, and select to sync those too. I think this may be my most used & favorite Snow Leopard feature.