Google has a very nice search engine for scientific papers, namely scholar.google.com. The only problem is that a lot of search results contain links to services that are not free. I believe information should be available for free, so this is what I do against this problem:

  1. Search for something, e.g. multi-agent systems.

  2. Whenever you click on a paper that does not lead anywhere except to a site asking you to spend your precious money, ignore this server in your search parameter, like that: -inurl:acm.org.

  3. Repeat until you are happy.

I usually have this ignore list:

-inurl:springerlink -inurl:kluweronline -inurl:ieee.org -inurl:acm.org

Now you get the search results that really matter.