http://www.urduseek.com/ : you can look up words in both English and Urdu, Urdu is unicode enabled.
http://www.urduword.com/ : less word than urduseek.com but has other features like "word of the day", put their search on your website, search for word using your mobile phone.
http://www21.brinkster.com/urdudict/default.asp : if you don't have access to Unicode Urdu and would like to access using "Roman Urdu" then try this one.
http://dsal.uchicago.edu/dictionaries/platts/ : Platts is one of the oldest Urdu dictionary around. If you are looking for some old Urdu words then this is the place where you can find them.
http://www.oup.com/us/catalog/general/subject/?view=usa&ci=0195793404 : if you rather have the dictionary in your hands then try this Oxford English-Urdu Dictionary compiled by Shanul Haq Haqqi.
http://www.ijunoon.net/urdudic/index.asp : another English to Urdu Online Dictionary.
enjoy your searching and give me your feedback on which ones you like and why?