CiteSeer is a search engine that makes it possible to search in many computer and information science papers and other publications. Most of those papers can be downloaded in PDF format.
In my spare time I like to study applications of neural networks and genetic algorithms, and CiteSeer is able
to find quite a lot of papers on those topics. But also a lot of scientific information regarding other
computer algorithms, techniques, etc. are easy to find. I use this search engine now and then and
even added a keymark to Mozilla Firefox
to make it possible to search from the address bar by just typing cs followed by the query.