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

JewishGen Gazetteer
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 or containing a specific string of letters.  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.

Kartenmeister
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.

Słownik Geograficzny Królestwa Polskiego i innych krajów słowiańskich
Published in multi-volumes in the late 1800s, this geographical dictionary of places in the Kingdom of Poland and other slavic lands is an excellent resource for finding places that may have become part of Belarus, Lithuania, or Ukraine or those that no longer exist today.  The complete content of these volumes is available online as scanned images.  The site is in Polish.  Click on Tekst slownika to access the content of the books.

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.