My single favorite improvement in Snow Leopard is the overhaul to system-wide Services. Services were one of the best features of the NeXTStep OS, and while they made the transition to Mac OS X, they never seemed well integrated into the user interface. From 10.0 through 10.5, the Services menu was hidden away in a sub-menu of the application menu, and keyboard shortcuts were not user-configurable (and in fact, it was common for multiple third-party applications to define Services menu items with conflicting shortcuts). The Services system has been completely overhauled for Snow Leopard.
Mac OS X Automation is a great new web site devoted to AppleScript, Automator, and Services, with examples and tutorials from the one and only Sal Soghoian. Their write-up of the changes to Services in Snow Leopard is the best you’ll see, emphasizing four C’s: Contextual, Convenient, Configurable, Customizable.
Customizable is the best part. You can now create your own system-wide (or app-specific) services using Automator, AppleScript, and shell scripts. Check out the list of downloadable Automator services you can install (and modify).