KEGG (Kyoto Encyclopedia of Genes and Genomes) is a bioinformatics resource for linking genomes to life and the environment. It has information on virtually every known topic in molecular biology, and has numerous links to other databases as well.
A portal to gene-specific content based on NCBI's RefSeq project, information from model organism databases, and links to other resources. I use this to get information about important genes that show up in my papers.