I know of no good way to quantify the value of one person's contribution to science, or indeed any other field of intellectual endeavour. Nevertheless, people often try to do so, and the vast majority of these efforts focus on citation counts. There are many suggestions as to what is the most reliable indicator: total citation count, average citations per paper, or something slightly more sophisticated, like the h-index. Needless to say, all of these have major flaws, and tend to favour scientists who have simply been around for a long time, or who work on popular topics, but I get the impression that such quantities are still sometimes used as an aid in decisions on hiring, tenure etc.
Such issues have been discussed exhaustively on the blogosphere and elsewhere (although I'm too lazy to gather any links), and I don't really have anything new to add. But I did want to point out that, as far as I can tell, accurate citation data often simply aren't available. In high energy physics, we are lucky to have INSPIRE, which does a remarkably good job of collecting and organising citations from the literature on particle physics, string theory, gravity, and related fields. This is helped enormously by the fact that the vast majority of such papers are posted on the arXiv. But even this isn't foolproof; some of my papers, for example, overlap with the interests of pure mathematicians, and have been cited by papers which aren't included in the INSPIRE database. Most of these (that I know of) have been picked up by Google scholar, but that service misses some citations which INSPIRE finds.
Here are my own papers, with citation information, on three different free services: INSPIRE, Google scholar, and ADS. You can see that things generally agree reasonably well, but in some cases there are large discrepancies. Since I don't know the source of these, it could be that other authors are affected much more dramatically. I have no idea what the situation is like in completely different fields.
It is nice to be able to keep track of one's citations (I think most people keep an eye on theirs), but there doesn't seem to be a reliable way to catch them all.