Resources at NIH and National Center for Biotechnology Information (NCBI):

Entrez is the integrated, text-based search and retrieval system used at NCBI for the major databases, including PubMed, Nucleotide and Protein Sequences, Protein Structures, Complete Genomes, Taxonomy, and others.

Of particular educational interest is the NCBI Bookshelf, a growing collection of biomedical books with fully readable content online and that can be searched directly.  For example, Harvey Lodish Molecular Cell Biology (4th edition) is downloadable or searchable; although this is an older version than currently published (2000), it is useful.

The first book to be made available at NCBI was Molecular Biology of the Cell, 3rd edn, by Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson, published by Garland Publishing, Inc. Molecular Biology of the Cell is one of the most widely used undergraduate textbooks in molecular and cell biology.