“pSearch is a customized search engine dedicated to philosophy-related web pages. It is powered by Google Co-op". The search engine is no longer being maintained by Joe Lau at the University of Hong Kong.
Use the taxonomy browser to find information on philosophical concepts at the Stanford Encyclopedia of Philosophy, Noesis, and Google Scholar web sites.
This website is a directory of online philosophical articles and books written by academic philosophers which are organized into categories. This directory can be browsed and searched.
The Bielefeld University Library maintains BASE (Bielefeld Academic Search Engine) which searches academic open access resources. A search for Plato returned over a 1,000 results.