Rogue "flushing' water pipe
If this pipe is just above ground level and is spewing clean water out onto the ground (or is otherwise unchecked), it is more than likely coming from a sump. This sump could be from a drain tile or the A/C drain. If you have access to the basement, this can easily be determined by tracing the pipe back from where it goes through the wall/structure, unless it is all behind sheetrock. 

(NOTE: I didn't see Tester101 and Fiasco Labs comments prior to writing this. I'm basically agreeing with them.)