You sure can! If you own Reign of Giants, Shipwrecked, and/or Hamlet and would like to play the original game, go to the game's main menu and do the following:
- Click on the "Play" button in the main menu;
- Choose a save slot that you'd like to use; choose "New Game";
- On the "New Game" menu, you'll find a check box for "DS", "RofG", "SW", and "HAM"
- Choose "DS" and disable all of the other boxes.
- Click on "Start" to play the game.
Doing the steps above will disable all of the expansion content and allow you to play the original, Don't Starve game.
*Please note that you will have to start a new game on a new save slot or overwrite an existing game in order to play the core Don't Starve experience.