Try these tables comparing the features of different search engines. Avicenna - A relatively new site geared towards health professionals. Medscape - Some features require registration.
Some features require registration.
Document retrieval by mail or fax for a price. WWW searches are free, information from proprietary databases aren't. Costs range from a few cents for each abstract retrieved to substantial to order the full text version of articles from some databases. If you're looking for physics information this is a good place to start. Although Yahoo is more accurately categorized as an index, the size of its index combined with the power of its search engine merits its inclusion here as well.