Expasy is Swiss bioinformatics resource portal providing access to databases and software tools from a range of life science including genomics, proteomics, system biology etc.
Uniprot is high-quality and freely accessible resource of protein sequence and functional information. detail tutorial:
"National Center for Biotechnology Information Protein" The Protein database is a collection of sequences from several sources, including translations from annotated coding regions.
SMS: The Sequence Manipulation Suite - lots of small programs in JavaScript for various sequence manipulations (Molecular weight, Isoelectric point, statistics, range extractor etc.)
PeptideCutter: predicts potential cleavage sites cleaved by proteases or chemicals in a given protein sequence.