just a bike really. anything else is extra.
Fenders, you only need if you think you will ride on streets if they are not dry or during rain/snow.
Lights, you only need if you ride at night or when visibilty is poor
racks you only need if you have things to carry and don't like to use back-pack or msg bag.
spare tubes/tools are really only necessary if a) your commute is longer than 1 mile/ or if you like pushing your bike or have transportation alternatives in case you have mechanical problems.
Special clothing only becomes necessary if your commute is longish (over 6 miles, I'd say - others may have different standards)
Lock is only necessary if you don't have an other way to secure your bike at work/or wherever you bike to.