Kartenmeister provides the spelling of the name of the city in both Polish and German (in this case, the same), as well as the German and present-day provinces. A link to Google maps shows the location and coordinates. There is also information on parishes in the area and where civil registration was held. I was able to use the parish information to order microfilmed church records. The entry even provides the population (314 people in 1905) and any local landmarks.
Besides the research value, Kartenmeister has the potential to connect you to other researchers. There is a list for each city of surnames and the email addresses of those researching them. You can be added to the list, so that if anyone else is researching the same surname in the same location, you can contact them. The surname list is also searchable for all locations.
“Information on German City of Sielec,” online database, Kartenmeister (Kartenmeister.com : accessed 12 May 2012).