If you use Time Machine (and it was just the QuickTime Player app that you deleted), open a Finder window to your Applications folder (and have it in front). Click on the Time Machine icon in the Dock to go to the flying through space interface. Go back in time until the QuickTime Player app is present. Select it and click Restore.
If you can't do the above, manually installing the latest Combo update for your version of Mac OS X may reinstall QuickTime as part of installing everything that has been updated since the inital release. Be sure to back up your data before doing a system level update, in case something bad happens.
The 10.7.3 combo update for Lion is here
http://support.apple.com/kb/DL1484
If your Mac OS X release is earlier, you can search for the latest combo update for Snow Leopard, Leopard, etc.