Supposing my opponent has little time left and I have much more than him (e.g. 10 minutes to 50, or 1 to 5, or 20 seconds to 1 minute).
What are good strategies that take advantage of my rival's time situation?
I read this article, and I will quote this part which I liked:
When the opponent is in time trouble, Webb advocated the barrage technique which involves planning two or more moves ahead, and then playing them in rapid succession. The idea is to give the opponent little time to prepare for the second move, increasing the probability of that move being a mistake. Playing a barrage of moves does increase the chance of a blunder from the barrager as well, and the technique is inadvisable in a winning position where a player should focus on winning on the board.
I agree that you should focus on your own game, but taking into account the opponent's time may help you. One idea could be to delay exchanges to make the game longer and make complicated positions on the board.
Edit: I'm playing blitz/rapid chess with times between 3 and 30 minutes, without any increment, and online.