MEDLINE is a great first stop for most researchers in the medical and allied health fields. It has full text for over 2,200 journal titles related to these fields and citations and abstracts for thousands more (use ILL to get the full text). You can also use advanced search features, like MeSH subject headings, to quickly identify the articles that have been tagged with the subject you are researching. In addition, MEDLINE allows users to refine their search by population and specify types of publications they are most interested in (e.g. clinical trials, meta-analyses, validation studies, etc.).
This database is especially well-suited to researchers looking for types of articles classified by the AMA as "Reports of Original Data."
The following databases are good alternatives to MEDLINE when searching for reports of original data like case studies, clinical studies, randomized clinical trials, etc. PsycINFO has a decided emphasis on psychological literature. ERIC is geared toward educational research.