(aka Ethanedioic Dehydrate Acid) is (COOH)2, which looks something like this:

 OH   OH
  \   /
   C-C
 //   \\
 O     O

Melting point:189° C
Oxalic acid is the only possible compound in which two carboxyl groups are joined directly; for this reason oxalic acid is one of the strongest organic acids.

Unlike other carboxylic acids (except formic acid), it is readily oxidized; this makes it useful as a reducing agent for photography, bleaching, and ink removal. Often used in wood and fiber bleaching.

couldn't find much more useful right now