The g-index, proposed by Leo Egghe in 2006 aims to overcome a bias against highly cited papers inherent in the h-index. The g-index is the "highest number of papers of a scientist that received g or more citations, on average". It gives credit to researchers who have published landmark papers.
The m-index, also proposed by Hirsch, is defined as h-index divided by the number of years since the researcher’s first publication. The index is meant to normalize the h-index so that early- and late-stage scientists can be compared. The m-index averages periods of high and low productivity throughout a career, which may or may not be reflective of the current situation of the scientist.
The e-index aims to address the number of “excess” citations above and beyond the h-index. The e-index is defined as the square root of the sum of the “excess” citations in the papers that contributed to the h-index.
Source: Benchfly weblog, 20 Oct 2010