Public databases

UCSC Genome Browser: Gateway for browsing many genomes (and BLAT, to map your sequence on the genome).
Ensembl genome databases and browsers for human, mouse, and other species.
NCBI Genomes including human and mouse
GeneCards - database of human genes, their products and their involvement in diseases
NCBI Entrez Genome Project
Online Mendelian Inheritance in Man (OMIM) via NCBI's Entrez browser
TIGR Gene Indices (multiple species) - a non-redundant view of all genes created from EST clustering and consensus building
UniGene - Unique Gene Sequence Collection for model organisms.
NCBI HomoloGene: a system for automated detection of homologs of several completely sequenced eukaryotic genomes
Mouse Genome Informatics - comprehensive databases of genetic information on the laboratory mouse
Gene Expression Database for the Mouse - at Jackson Laboratory
Zebrafish WWW Server Home Page

Wormbase
C elegans Database and Blast server - at the Sanger Center
SmedDb: Schmidtea mediterranea (planarian) Database
FlyBase - Database of genetic & molecular data for Drosophila from Indiana University
Berkeley Drosophila Genome Project
SGD: Saccharomyces Genome Database
Saccharomyces Genomic Information Resource - at Stanford University
Candida Genome Database: genomics of Candida albicans

NCBI (National Center for Biotechnology Information): resource for databases like PubMed, Entrez, UniGene, dbEST, OMIM, and many others (many of which permit download of data archives)
The NCBI Handbook: descriptions of NCBI databases and how to use them
DDBJ: DNA Data Bank of Japan
EMBL Nucleotide, Protein, and other databases
EBI: the European Bioinformatics Institute; manages many databases and analysis tools