CiteSeer http://citeseer.ist.psu.edu/pop
Search engine for computer science and related matters. For all results number of citations are given which allows to estimate the importance of a paper in a given context. Most of the papers are cached and available for direct download as postscript or PDF.
ISI Highly Cited http://isihighlycited.com/pop
A search tool to identify individuals, departments and laboratories that have made fundamental contributions to the advancement of science and technology in recent decades.