It occasionally happens that some liquid sits on top of yoghurt in the container, and many people prefer spilling out the liquid before eating the yoghurt. At first glance, this would appear to be forbidden on Shabbat, as it entails removing Pesolet (an undesirable substance) from Ochel (the desirable substance).
In truth, however, many Halachic authorities permit pouring out the liquid from the top of the yoghurt. Hacham Bension Abba Shaul (Israel, 1923-1998) writes that when one pours out the liquid, we may consider him as removing Ochel from Pesolet, and not Pesolet from Ochel. By tipping the container in such a way that the yoghurt remains, one essentially removes the yoghurt from the liquid, and thus it is permissible. Hacham Ovadia Yosef, in Hazon Ovadia – Shabbat (vol. 4, p. 229, in the annotation; listen to audio recording for precise citation), advances a different argument, claiming that the liquid and yoghurt are not actually mixed together, and thus removing the liquid cannot be considered "separating." Since the liquid sits on top of the yoghurt, and is not mixed together with it, removing the liquid would not constitute Borer and is thus permissible on Shabbat.
This would apply as well to one who wishes to pour out the liquid from a can of olives or pickles. Here, too, one could argue that the person is separating the olives from the liquid, and not the liquid from the olives, and, furthermore, the liquid and olives are not actually mixed together. Hence, Hacham Ovadia Yosef ruled (in Hazon Ovadia, ibid., and in Yalkut Yosef, in the laws of Borer) that it is permissible to pour the liquid out of a can of olives or pickles on Shabbat. He noted that the work Shemirat Shabbat Ke’hilchatah (by Rabbi Yehoshua Neubert, contemporary) forbids pouring the liquid out of a can in such a case, but in the 5770 (2010) edition of the work the author retracted this view and ruled leniently. This is, indeed, the Halacha, and one may pour the liquid out from a yoghurt container or can of olives and the like on Shabbat.
Summary: If there is liquid sitting on top of a yoghurt container, one may pour it out so he is left with only the yoghurt. Similarly, it is permissible to pour the liquid out from a can of olives or pickles.
