http://mapa.szukacz.pl
This site, which replaced www.pilot.pl, is in the Polish language, but you don't have to understand Polish to use this great site.  Enter the name of the town in the data box under "Miejscowość" and click on the "Pokaż" button.  As well as a map, you will also find the province (woj.), county (powiat), and district (gmina) it belongs to, and the zip code (kod.).

JewishGen's ShtetlSeeker
Search for towns in Central and Eastern Europe using present or former names.  You can search using exact spelling or the Daitch-Mokotoff soundex system, or you can find town names beginning with a particular spelling.  A truly wonderful tool!

Former German Town Names and their Present Polish Names
This is an excellent list of former German town names and their present Polish names.  There are two lists: one lists the German names first and the other lists the Polish names first.

Konkordanz der geographischen Namen (German/Polish villiage names)
Another excellent list of former German town names and their present Polish names.  There are two columns: one lists the German names first and the other lists the Polish names first.

Database Search Utility
An excellent resource containing more than 52,000 former German town names and their present Polish, Russian, or Lithuanian names.  The information also includes the former German county name, former German province it was located in, and the current province name.

Directory of Cities, Towns, and Regions in Poland
This is a good site to use if you're deciphering handwriting or working with mispellings.  It can be very helpful when you can't quite read the whole village name but are pretty sure of the first two letters.