Unfortunately Lunfardo English Dictionary websites are not common on the internet (for a quick introduction to Argentina Lunfardo check out this What is Lunfardo? article.) Here is a list of some good websites that provide detailed information of Argentina lunfardo and slang. There are sections for Argentina-English websites as well as Spanish-only websites.
Lunfardo English Dictionary websites
Speaking Latino includes an extensive list of Argentina slang words and phrases but does not separate traditional Lunfardo terms from other Argentina Spanish terms. (Speaking Latino home page)
FreeLang.Net includes a HUGE collection of dictionaries that may be downloaded for free and used directly on your computer. Included is a Lunfardo English Dictionary with a few thousand words.
Living In Argentina publishes a series of Lunfardo-English word lists, defining each word or phrase in English. The first two posts are here:
Taringa.Net is a dictionary of almost 2500 words. (Taringa)
Lunfardo Spanish-Only Websites
These websites are Spanish-only websites. Even if you do not speak Spanish fluently you may search for a word and copy its definition into an online translator such as Google Translate, to check the word's meaning.
El Portal Del Tango with a Diccionario de Lunfardo that includes thousands of words with their definitions. (Diccionario de Lunfardo)
ElCastellano.Org includes a list of Argentina Spanish terms, definitions and in some cases their equivalent word used in Spain. (ElCastellano.org for Argentina)
Babylon Lunfardo English Dictionary is a searchable dictionary with almost 3000 entries. You may also download the dictionary and use it with their software. (Babylon Lunfardo English Dictionary)
Check out these other Argentina Spanish Slang Dictionary articles.