MAC Operating System Question:

Try pressing Option-Command-Escape. (press them in that order, but don't release any of them until you've pressed the last one.) This will give you the option to force quit any crashed applications. However, force quitting an application will cause any unsaved changes to be lost. Your best bet is to wait a minute or so, and see whether you can quit any applications normally. If you can't, save what you can, and restart your computer.
Important note:
Before you press the Option-Command-Escape or use the equivalent command that you find in the Apple menu. (in the upper left of the menu bar) If all is unresponsive and you're seeing a "spinning beach ball", try to be sure you're in the "Finder" by making a click anywhere in a free space on the desktop or pressing the Apple key (= command) and the Tab until you'll see the Finder's icon (and all the rest of the open applications). Select it either by using the arrow keys or keeping pressed the Apple and pressing again Tab (or moving the cursor on it and releasing it).

There are also other ways if the force quit command fails.
You can open an application called Activity Monitor by searching for it in spotlight. It will show you a list of all open applications, and give you information about the amount of processor power, memory, and virtual memory being used. If a program appears to be using too much CPU or memory, or is highlighted in red, you can quit it from within Activity Monitor by selecting the program, and clicking the quit button in the toolbar. This will give you the option of a standard quit, or a force quit. Try the standard quit first, but if that doesn't work, use the force quit option.
The following method utilises Mac OS X's command line utility.
Keep the application named Terminal in the dock (you can add it dragging its icon on it).

Open it and type "top" (of course without the quotes),you'll see a list of all the open applications,if you know which one is causing the problem look at its "name" ,its ID is a number on the left,open one more terminal's window and type "kill" (it's not a joke it's a command that is called in that way) followed by that number and press the enter key (the last one on the lower right of the numbers under the plus sign) or the return key (the second biggest one after the space bar).
Some time when the first method fails this one works.
If all this is not enough just wait,if you're lucky the spinning ball will become again a cursor after some time and you'll be able at least to save all before to restart.

If the problems return try adding all the RAM you can,Mac OS X 10.6.1 Snow Leopard is very stable thus perhaps you're keeping too much applications open at the same time.
Remember that on a Mac to quit an application you've to be sure that it's in the fore ground and press command (Apple key) and Q.
In other words to close the last window of an application doesn't quit the application itself.