I use Google Scholar Alerts. Alas, they are far from optimal. First, Google has an epically flawed system of recognizing the names of the authors from a paper (so a paper about cyclic homology will more often be shown as authored by "C. HOMOLOGY" rather than by its actual author). Second, they do a bad job at filtering. My "Witt vectors" scholar alert spews out loads of epidemiology and medicine texts due to Witt being a not-too-rare surname and "vector" also meaning a way a disease is spread. (This even got some of the Google Scholar Alerts I receive automatically sorted into the Spam folder by Google's very own GMail spam filter. Apparently the culprit was a paper containing "penis" in its summary.)

Still, out of all ways to obtain digests of new research activity, this seems to me the best one at the moment.