H-index = scholarly impact
A scientist has index h if h of [his/her] Np papers have at least h citations each, and the other (Np − h) papers have at most h citations each.
The H-Index was developed by Jorge Hirsch in his quest to find a better way to rank authors within their field.
As described in the linked article from Wired Magazine, H-Index rankings do seem to generally mirror scholarly success, but with an advantage to authors who have published longer.
Since publishing output varies by discipline, authors' H-Index rankings should only be compared within a discipline.
Resources to access the H-Index are listed below:
With the altmetrics movement, there have been many new sources and information products that compute impacts of different kinds. The following products are certainly worth keeping on your radar: