Biology 

From the NCBI website: "Established in 1988 as a national resource for molecular biology information, NCBI creates public databases, conducts research in computational biology, develops software tools for analyzing genome data, and disseminates biomedical information..." You can find instructions for using these closely linked databases here.

Entrez Nucleotide
A collection of sequences from several sources, including GenBank, RefSeq, and PDB.

Protein sequences
Compiled from a variety of sources, including SwissProt, PIR, PRF, PDB, and translations from annotated coding regions in GenBank and RefSeq.

Genome database
Provides views for a variety of genomes, complete chromosomes, sequence maps with contigs, and integrated genetic and physical maps.

Molecular Modeling Database (MMDB)
Contains over 40,000 3D macromolecular structures, including proteins and polynucleotides. It's linked to the rest of the NCBI databases, including sequences, bibliographic citations, taxonomic classifications, and sequence and structure neighbors.