One possibility is Highwire at
http://highwire.stanford.edu/lists/freeart.dtl
Another is finding abstracts at Pubmed or Scirus
http://www.scirus.co
and going to the websites of the cited journal, sometimes they have full free text avaliability.
The US patent Office, at
http://www.uspto.gov has citing and summations from applicants.
The FDA and HHS, many subsites, have drug and clinical trial data, some full text access.
A metasearch engine, I like Ixquick, has links to web sites that provide full texts, generally to suport commercial selling.
In depth research will probably require $ for online access and/or a local sci/med library, one can only get a limited amount for free online, it's relatively easy to go broad , but not deep.