Medline is one of the key databases, and it is available in two ways:
TechCat includes both Medline and PubMed Central, and also offers citations from 70 different databases with direct linking to full text provided from many more resources.
PubMeds the searchable version of Medline and includes PubMed Central, which is made up of selected full text. PubMed has some additional search types and limiters to provide more in-depth searching.