Some Scrabble® Resources Moved
The blurb "About Scrabble®" and various Word Lists have migrated to the DC Scrabble Club's website. There is also the section on Sources and Study Tools there.
But here are some of the key links:
Computer Games
Jason Katz-Brown and John O'Laughlin announced on 6 March 2006 "the public release of Quackle (www.quackle.org), a world-class crossword game artificial intelligence and analysis tool. It can be configured to play and analyze SCRABBLE® brand crossword games and use the newest lexicons. Quackle includes a move generator, move evaluator, simulator, and Qt-based user interface and can be used with any board layout, alphabet, lexicon, and tile distribution. It is licensed under the open-source revised BSD license." I.e. it is a free way to simulate best play so you can analyze your game and find out what your weak points are.

The Internet Scrabble Club allows you to play scrabble online (for free). For a good reason to dislike the Facebook application, see this NPR piece which mentions a game I played against Stefan Fatsis.


Word Lookup
Zyzzyva and LeXpert are excellent tools for studying or looking up words (NOTE: you don't want to use Hasbro's site to look up words, since it will fail to find inflections that are listed in the body of the definition, e.g. RANGY vs. RANGIEST). Though Zyzzyva is still in the earlier stages of development, it includes definitions, which is a boon to beginners (serious players tend not to care at all about definitions). Carol Ravichandran's LeXpert program has been around for much longer, but Carol Ravichandran announced Jan 28, 2007 11:25 AM that no future updates were likely.
Cheat Sheets
Rebecca Slivka of the Seattle Scrabble® Club has made a useful pair of cheat sheets (or one sheet in two parts: part 1 and part 2) for learning the most important new words. There are also two PDF files designed to be double-sided one-page cheat sheets for beginners here: one with just 2- and 3-letter words and Q w/o QU words, and one that adds 4- and 5-letter J, X, Q, and Z words.
Other Resources


OWL Errata
Note these corrections to the first edition of the recently published Official Word List (aka OWL2, distinct from the OSPD in that it includes bowdlerized words and excludes definitions), posted by Jim Pate:

Additions:
page 98 CRABAPPLE
page 98 CRABAPPLES
page 118 DEVELOPPES
page 186 GODDAMNDEST
page 279 MUNCHABLES
page 485 VIREONINES

Deletions:
page 98 CRABABBLE
page 98 CRABABBLES
page 186 GODAMNDEST
page 491 WEAPONEERED


OSPD Errata
The Official Scrabble Player's Dictionary (most recent edition is the Fourth, aka the OSPD4) is a lamentably inaccurate document, and it excludes these "expurgated" words which are acceptable in tournament play (except when televised by ESPN).

Rebecca Slivka created a printable "errata" sheet for OSPD4 that includes all new expurgated words, all words accidentally omitted from the OSPD4 printing, and enumerates the deletions. If you combine that document with the OWL1 expurgated word list, found at Steven Alexander's website, you should have all the differences between OSDP4 and OWL2.