This is a short list of effective search engines for use on the Internet as a whole and for general science topics. One important thing to realize about search engines is that they all operate on different principles, use various algorithms, act upon a wide range of databases of various sizes, and each has its own particular strengh. There are ways to access some of the stuff on the web that the general search engines do not pick up. Searching the invisible web discusses the problem and points to some tools to help solve it. Note that there are two types of searching methods- using a search engine to parse keywords through an algorithm, and browsing through more and more finely classified subject lists in a tree type structure.
Many search engines allow both kinds of searching.
The two types are each suited for different kinds of queries and documents. Engines vary in the level of sophistication allowed in the query. The newest search engines can use natural language and use the number and type of links TO and FROM sites to judge relevance. There are also sites that do not purport to be comprehensive search tools, but rather are selective, sometimes subject oriented, guides. Search or browse by categories.