Hydrogen sulfide is a chemical compound made up of two atoms of hydrogen and one of sulphur (H2S). It's a gas at room temperature. It's also highly toxic. It's commonly formed when organic compounds containing sulphur decompose in the absence of oxygen. Its smell is characteristic of decomposing egg yolks (which are rich in sulphur) and provides most of the odor of flatulence (which is mostly odorless methane).
One of the pranks Lisa Cuddy plays on House when she takes over half his office in Let Them Eat Cake is to spill some hydrogen sulfide on his desk just before she leaves for the day.
In Two Stories, House releases hydrogen sulfide into the clinic to clear it out so he won't be seen stealing Cuddy's laptop from her office.