Here is a brief list of those medieval philosophers I myself am aware of, so it is certainly not exhaustive:
St. Augustine of Hippo
St. Augustine (4th-5th century) is borderline medieval, but very crucial to an understanding of much of medieval philosophy (since theology dominated during this period). Augustine's Confessions is a work of metaphysics and Christian theology that had (and continues to have, to a lesser degree) a very prominent influence in proceeding philosophy. Augustine himself is the leader of an entire monastic movement, and so also has a strong religious influence today.
Anselm of Canterbury
St. Anselm was another quite prominent Christian philosopher, although he came much later than Augustine - the 11th century. Anselm is probably best credited for the ontological argument for God (referenced by later philosophers Descartes and Kant), a very influential argument that has been both widely supported and objected to.
Maimonides was perhaps the most influential Jewish philosopher, and lived in the 12th century. His works included significant interpretations of the 613 Jewish commandments, and an analysis of the concepts entailed by monotheism. He also focused his philosophy on creation, and developed a theory with classical influences. His philosophy, too, has been very influential in theology and metaphysics.
Ibn Sina (also known as Avicenna) was one of the most prominent Muslim philosophers of the Medieval era, and also lived in the 12th century. Although it is difficult to point out a specific Islamic thinker that dominated this period, Ibn Sina is likely one of the more significant influences. Like other medieval philosophers, his extensive works often covered metaphysics, but from an Islamic point of view. Much of his philosophical pursuits focused on the reconciliation of rational philosophy with religion (a very relevant issue, soon made apparent in the Renaissance). His other influences include law and medicine.
Again, this list is in no way complete; I have just listed those philosophers I know of.