PubMed is one of the most popular and well-used databases to search for medical, health, bioinformatics, and genetics literature; which includes MEDLINE. Tutorial available here.
Favorite Features:
MeSH
Citation Matcher
Clinical Queries & Clinical Trials
Similar Articles
My NCBI (saves search history and allows for search alerts)
International multi-disciplinary indexing & abstracting database for scientific, medical, technical, social sciences and arts and humanities. Includes tools to track, analyze, and visualize research.