For my undergraduate research-based courses, I used a lot of peer-reviewed articles, some of which came from PLoS, predominantly PLoS One, PLoS Pathogens and PLoS Medicine. Those articles were at least 1.5 years old and I found them through other databases that aren't open access. Many journals have their own websites, although again not necessarily open access. It's interesting to see PLoS is open access, so I'll be browsing around there a bit more, especially for articles written in 2012, especially for pathogens and neglected tropical diseases as there are a few diseases that gained some attention, so I'd like to see how they were handled.