Consider two large flat plates placed near one another. The plates are parallel, and have equal and opposite charges uniformly distributed. This configuration is known as a parallel-plate capacitor.
The net field is the vector sum of the individual fields, which have the same magnitude. In this situation there is a uniform field between the plates, and zero field everywhere else. The field between the plates is twice the field from one plate, so the net field is:
E = s/eo
A parallel-plate capacitor is a great way to create a uniform field. The field is perfectly uniform only when the plates are infinite in size, but as long as the width of the plates is large compared to the distance between the plates the field is fairly uniform.