The cactus is the must-have houseplant of the moment, and your desk will be all the better for one.
Cacti need six hours of full sunlight a day. (You know, like in the desert.) That means they should be placed close to a west- or south-facing window. Rotate your plant occasionally to ensure each side gets its fix.
Desert lore has it that a sliced-open cactus can provide emergency hydration. In fact, drinking from one will make you quite sick. But it is true that cactuses store a significant water reserve, so during the spring and summer growing season, allow soil to dry completely before watering.
Most cacti enter a dormancy period during fall and winter, when they prefer cooler temperatures and less water. Move them away from radiators or heaters, and cut back to a light watering once a month.