For rapid, effective and entirely free (to the user) indexing to open access medical literature, PubMed can't be duplicated or surpassed. If you need to limit your search to medical articles that are freely available, PubMed is simply the best. Do any search and notice on the far right of the results screen a link to "Free Full Text" -- there are many other limits and very powerful search options, but this is one of the most obvious and helpful for the public.
Medline, the database underlying PubMed, can be simultaneously searched with Chemical Abstracts in SciFinder; with BIOSIS and the ISI citation indexes in Web of Knowledge; and with a whole host of databases in EbscoHost. There are excellent reasons to search Medline in those different platforms, but none of them are freely accessible by the public, world-wide, and offer the same ease of quickly linking to free articles online. PubMed: our tax dollars at work, and worth every penny. Try the advanced search interface with its "search builder" and be amazed at the options for creating a well-focused search. PubMed Mobile provides access from just about anywhere, anytime. Thank you, NCBI.