MathSciNet is a "comprehensive database covering the world's mathematical literature since 1940." It indexes some 1,800 journals and provides signed reviews of the articles. It also includes references to books and conference proceedings. While the focus is on mathematical literature, it also indexes materials from other sciences, including Computer Science, Engineering, Economics, Physics, and Biology.
Content: Citations, plus reviews of journal articles, with links to full-text articles. Note that access to the articles themselves most often requires a subscription. Also includes references to book and conference proceedings.
Coverage: 1940 forward