I find myself using the Application Switcher function of Mac OS X dozens of times each hour during the work day. If you haven’t tried it yet, you can view it by holding down the Command key while repeatedly pressing the Tab key.
There’s more to the Application Switcher than just switching applications. You can quit open applications by selecting the program in the Switcher and pressing the Q key. If that application is frozen, Application Switcher will instead force quit it (most of the time at least), saving you a trip to the Apple menu.
You can also drag a file to a running application in the Application Switcher. Say you have an Excel spreadsheet that you want to open in Numbers instead of Excel. Because Excel files open in Excel by default, simply double-clicking on the file will launch Excel. With Numbers running, begin dragging the document and then press Command-Tab. Drag the file to the Numbers icon and the file will open up in that program instead of Excel.