Rabbi Yitshak Kaduri, shlit"a, the 108-(or more)-year-old rosh hamekubbalim (head of the kabbalists worldwide), is reported to be seriously ill in a Jerusalem hospital. It is proper to pray for his recovery, and to say tehillim (psalms) and do extra mitzvot in his merit. Indeed, Chief Rabbis of Israel have called for Jews worldwide to pray for Rav Kaduri's, shlit"a, recovery, and Rabbi Ovadia Yosef has gone to Bikur Holim hospital to visit him.
Rav Kaduri's name is Yitshak ben Tufaha. (For online resources, including tehillim in English and in Hebrew, see this post.)
UPDATE: 1-16-06 The Jerusalem Post is reporting that Rav Kaduri, shlit"a is in critical and unstable condition, and has now been put in a medically-induced coma.