An constantly updated, online database of biomedical science articles published in science journals. Operated by the National Library of Medicine and the National Center for Biotechnology Information, accessed by the latter's Entrez data retrieval system.
It's hard to explain how much PubMed has accelerated biomedical science. In the past, one had to pull journals out of the library, and find journal articles that one article would have referenced. Now, if you want to find out about Adult T-cell Leukemias, you point a browser to "http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed" and search on the term. You'll get all the articles published that has any relevance to Adult T-cell Leukemias, even currently unpopular studies that may turn out to be right in the future.
Basically, it makes sure people have access to almost all published research, where in the past, you would have had to find out about stuff not published in the mainstream journals in some other way...
It's gotten even more useful in the last few years, because many journals now have on-line versions where you can link to the actual article itself from the article's PubMed entry, saving you the time of going to the library to read or copy the article.
One of the many government run internet services that are actually useful, like the IRS website.