Free = good.  Add in easily-available-on-line and you have better. If you have a need for medical research, follow the link to the LLRX.com article above and find a wealth of health-related sources, including journals, textbooks, rankings and images and gateways. There are useful descriptions of how to use the resources as well. Enjoy! 

Researching Medical Literature on the Internet – 2008 | LLRX.com