There are a number of problems associated with using search engines. Searches can be slow, search engines can have a strong American bias and a search usually generates some irrelevant leads. The quality of sites located using a search engine can be variable as the sites have not been assessed. The search engines listed below have useful features such as advanced search screens which allow you to construct complex search strategies. They are manually updated and maintained and are smaller than search engines.
Virtual libraries are useful because they tend to produce more relevant results.
BUBL covers all subject areas and is a very good site for finding information in the Sciences generally. Websites available through information gateways have been reviewed and evaluated giving the searcher access to high quality material. As with virtual libraries you have the option to perform a keyword search or browse the subject sections. There are a number of gateways based in the UK so American bias is less. Biome describes itself as Your guide to Internet Resources in the Health and Life Sciences.