The DNA methyltransferase (DNMT) inhibitors azacytidine and decitabine are the most successful epigenetic drugs to date and are still the most widely used as epigenetic modulators, even though their application for oncological diseases is restricted by their relative toxicity and poor chemical stability. Zebularine (1-(β-D-ribofuranosyl)-1,2-dihydropyrimidin-2-one), a more stable and less toxic cytidine analog, is another inhibitor of DNMT with concomitant inhibitory activity towards cytidine deaminase. Unfortunately, there is no new information related to the possible clinical applications of zebularine. Although many new inhibitors of DNMT have been identified, none of them can so far replace azacytidine, decitabine and, to a lesser degree, zebularine. This review summarizes the current data and knowledge about azacytidine, decitabine and zebularine, and their role in present and possible future epigenetic cancer therapy. We also discuss the molecular modes of action of these agents with consideration of their different toxicities and demethylation profiles, reflecting their complex and partially overlapping biological effects.