It search peer-reviewed papers, theses, books, preprints, abstracts and technical reports.
Some highlighted features:
- search from many online database
- provide direct link to downloading the paper
- "view as HTML", so you can read it right on the browser
- citation information (times + link to the citing papers)
- many versions of a paper available
- free service
Just try it and you will love it.
There're some advanced search syntax too, like
" author:witten string theory "
(Thanks Chris for introducing me this)