Nature Summaries of some but not articles, together with TOC for each issue which do not not seem to be complete in relation to printed version, and also a .pdf format index from past issues. Registration required, but free. Nature Medicine Free registration required, has search engine, archives to 1997 and abstracts. New England Journal of Medicine Abstracts of many but not all articles, plus editorials etc. Currently a free trial acess to on-line version of this major medicla journal.
Search engine allows many options, e.g.author, title, anywhere in text etc.
Also Medline access. Preventive Medicine Table of contents and abstracts for 1995-7 from the Academic Press Library service. This is one of the better on-line journal sites around. Best of all, they have an excellent search engine, not unlike those used to access Medline. It allows you to search for specific citations, authors, keywords in title or whole text.