Methionine (MET or M) is a hydrophobic amino acid. Its shape in Foldit is unique. It's shorter than Lysine.
Methionine occurs as the first amino acid in Foldit puzzles quite frequently. It has a strong tendency to form part of a helix, as part of the "MALEK" group of amino acids.
In the Foldit Lua interface, the functions structure.GetAminoAcid and structure.SetAminoAcid use lowercase "m" for methionine.
For more, see Methionine on wikipedia.