Reacts with water to form both sulfuric and hydrochloric acids. [link|http://www.dmcc.com/chlorosulphonicacid1.html|Link].
IIRC, I used it in organic chemistry lab in high school to make some red dye.
And also in that Chem lab was [link|http://www.encyclopedia.com/html/p1/picricac.asp|picric acid], a nice yellow dye. Also known as 2,4,6-trinitrophenol, C6H2(NO2)3OH, and a cousin to trinitrotoluene (TNT). Yep, it's an explosive. Talk about a "dual use" material.
It's incredible now to think high school students had access to this stuff.