You can search the contents of packages included in the various Ubuntu releases on the [Ubuntu Packages][1] website. Look under the heading "[Search the contents of packages][2]".

For example, here are the search results for libnss3.so in lucid (10.04):

http://packages.ubuntu.com/search?searchon=contents&keywords=libnss3.so&mode=exactfilename&suite=lucid&arch=any


  [1]: http://packages.ubuntu.com/
  [2]: http://packages.ubuntu.com/#search_contents