The most frequently cited paper, with 462 citations to date (again, using ISI data):
John B, Enright AJ, Aravin A, et al., 2004. Human MicroRNA targets. PLoS Biology 2(11): 1862-1879 e363. doi:10.1371/journal.pbio.0020363
Scopus reports 485 citations to this article, noted directly on the PLoS Biology site.
Far more exciting than simple citation analysis, though, is the potential for scholars and readers to interact, with options for commenting on and rating an article, jumping to related work, sharing the article through social networking tools, and finding similar research through guided searches in other databases. The PLoS article metrics includes the total number of page views for articles published after June 2005, giving another indication of the impact of open access.
The following article, for example, has been viewed 26,932 times:
The search and browse options on PLoS are easy to understand and highly effective; and the content is quickly found through Google Scholar, ensuring good reader access. I especially appreciate the Weekly Editors' Picks, which led to this article today: