After briefing the need to rollout to the end of Runway 17 to use the turnaround displayed on the commercial ATY airport page; the Captain discovered there was a parallel taxiway not shown on the chart.

Narrative

Prior to landing in ATY (Watertown; SD) I had briefed a visual approach to Runway 17. (I have not been to Watertown for several months.) Part of that was looking at the airport diagram. The diagram on the current chart (dated 6 FEB 09) shows that there is no parallel taxiway to Runway 17. It only shows a small turn-around area at the end of the runway. With this information; I planned on stopping on the runway and then back-taxiing; so that is what I briefed. Upon landing; we rolled out on Runway 17 and I saw there was a parallel taxiway to our right with feeder taxiways to the runway. I stopped on the runway and said; 'What the hell?' since I was shocked to see a parallel taxiway. For a brief moment; I wondered if we were on the wrong runway but then I quickly shook that off as I saw the runway number and we backed it up with the localizer back-course and the FMS. I looked again at the airport diagram and saw no parallel taxiway showing. Seeing that the taxiways were marked; painted; and plenty big enough; we exited the runway using the feeder and parallel taxiway and taxied to the gate without incident. I thought the lack of a proper map on the commercial chart was particularly disturbing because this chart was just updated last month. I know there is no way the parallel taxiway went up in only a month's time; so why has the chart not been updated? If this had been at night I think it really would have freaked us out. This is not proper and with the industry-wide focus on runway incursions; it is unconscionable that we don't have accurate airport diagrams. I don't know why chart company can't get these charts up to date. While I bring it up; Brainerd; MN (BRD) still does not show marked taxiways on the airport diagram. The map is accurate with the taxiways but they are not marked on the map even though they are on the airport itself. It makes it very difficult when a NOTAM is issued for a particular taxiway; yet we don't know which is which.Callback conversation with reporter revealed the following information: Reporter reinforced his concern regarding the lack of chart depiction for taxiway identification on airport pages when the airports do; in fact; have signs designating them.