The h-index (or Hirsch index) is a relatively new, but increasingly important, method of assessing the impact of an individual's publications.
This is an emerging movement to use a wider range of metrics in addition to traditional citations. This can include discussion (e.g. Twitter), saving (e.g. Mendeley, Delicious) and sometimes viewing or recommending an item.
This can give a better picture of impact, but until this is better understood probably needs to be used with caution.
We have a lot more information on our guide to altmetrics.
Essential Science Indicators allows you to identify the most highly cited (top 1%) papers in the Web of Science Core Collection. (Note the top 1% is a high threshold - many good papers will fall outside it.) It also includes Field Baselines (expected citation rates for papers in a research field).