Let's take Photoshop and Microsoft Office as examples. When you upgrade, let's say from Photoshop CS3 to CS5 or from MS Office 2008 to 2011, the old versions of your programs stay installed. This is how you can get rid of these programs, without creating too much of a fuss.
First, click on the Applications folder shortcut in your Dock. If you are running OS X 10.6 and later, it should open a black screen with all of your installed Apps. Earlier versions of OS X will open the folder directly. One way or the other, you'll have to click 'Open in Finder' at the end of the list of available programs. Once you've selected these, just manually drag and drop them into the trash. If you're doing things correctly, there will be a prompt that will ask you for confirmation (your user password) in order to complete this task.
For some programs, you'll have to right-click on the icon in Finder, and select 'Show Package Content'. You'll only have to do this if the application is part of a bundle and not self-contained. Then, move the files to the trash. You can free up quite a bit of space on your hard drive if you've got a lot of programs installed. If you drag and drop the Apps directly from the first black screen when you click Apps, it won't uninstall the programs.