NCBI/Nucleotide database
Focused on "genes". Gene
Focused on "SNP" (single nucleotides polymorphisms): Variation Viewer or dbSNP or Clinvar (database focused on SNPs in pathology)
Genecards (human) Atlas (oncology) Ensembl (vertebrate genomes) TCGA (cancer)
Sequence analysis
SMS: DNA stats, Filter DNA, Range Extractor DNA, Reverse complement