In Words with Friends, a Scrabble clone for the iPhone, slang words - such as the following - are permitted:
- za, slang for "pizza,"
- lez, slang for "lesbian,"
- And so on...
Are slang words allowed in official Scrabble games / tournaments?
You can check for yourself at the Official Scrabble Dictionary. According to that site:
Words With Friends uses the Enhanced North American Benchmark Lexicon with a few additions per their website. ENABLE is a public domain word list for use with word programs.
Thanks to @thesun's research it is obvious that the website is not the final word for Scrabble tournaments. Apparently the Scrabble folks censor their website, and books of around 200 offensive words.
To get the complete list you have to join the National Scrabble Association and pay for it. For most tournaments that will be the list used. If the tournament is televised then perhaps those 200 offensive words will not be allowed.
Slang words are 100% allowed in Scrabble - well, as long as they've been around long enough to have found their way into the dictionary.
The idea that slang is in some way verboten in Scrabble is mysteriously widespread among casual players I've run across. I defy them to find that prohibition in the actual rules!
"Za" is a fairly recent addition to the canon of official Scrabble words - I think it was popularized by the TV show Friends, or something? But it's now kosher, and a very welcome way of being able to quickly play a Z, especially in parts of the word that don't believe me when I tell them that ZO is a type of Himalayan ox. (Similarly, where would the game of Scrabble be without QI? Nearly unplayable, that's where.)
"Lez" may or may not be allowed the wordlist Pat had checked, but I'm pretty damn sure that some Scrabble wordlists have it in. (Mind you, I'm English, and the English official Scrabble wordlist is about twice the size of the American, so that could explain it.)
There was a televised world Scrabble championship of some kind which was notorious for one of the players playing "LEZ" in the final - and the judges quickly ruling it legal in spluttering embarrassment, due to the word's obvious potential to deprave and corrupt youngsters in the viewing audience. I'm pretty sure the implication was that LEZ was indeed in the wordlist: the guy was one of the top players in the world, and these people know their valid words. Afterwards he, quite rightly, said "I don't even think about what the words mean, I just play them if they're the best move". Happily, I do believe he went on to win the game and the championship!
EDIT: Here's a sample news article about the "lez" incident: http://news.bbc.co.uk/2/hi/americas/3542570.stm
In addition to the comments made by other users, it should be noted that the ENABLE+ list used by Words with Friends and the North American Scrabble tournament lexicon, the Official Tournament and Club Word List, 2nd Edition (OTCWL2) are practically identical for the vast majority of words used casual, beginner, and intermediate tournament play. The most notable omission from ENABLE+ is
JIVY, which is allowed in North American Scrabble, and the most notable addition to ENABLE+ is
ZEN, which is not allowed in North American Scrabble.
Also, you can easily obtain the full North American Lexicon by downloading the (free, open-source) program Zyzzyva, which has searchable versions with definitions of all of the various Scrabble dictionaries used worldwide. Several iPhone apps, including Checkword and Zarf. The tournament lexicon (as of July 2009) is no longer managed by Hasbro (the company that owns Scrabble), but rather by the North American Scrabble Players Association (NASPA) Dictionary Committee, and thus it is freely available through the sources mentioned above.
Use the Scrabble Dictionary for further reference. Slang words such as za and zn are recognized even if they don't seem to look like a word. In this lens - http://www.squidoo.com/scrabble-solver-words-that-begins-with-z - you will find more awkward words yet they are recognized.
David's suggestion of Zyzzyva looks like it might be the most comprehensive resource, but here are some others that might also be useful.
The Internet Scrabble Club has several lists to download, including SOWPODS and TWL06.
Scrabblist.com lets you verify words in either the SOWPODS or TWL lists. It has the advantage of not requiring any downloads. It doesn't give definitions, but Ninjawords is a very convenient tool for finding definitions.