Here are some of my favorite playgrounds:
The new Kid! Wordnik!
READING, WRITING, AND RESEARCH Grammar Girl's Quick and Dirty Tips.
English/German translation -- including metaphors and idiom!
Take Our Word For It (not active, but a great archive) Great news! As of January 2010, TOWFI is active again!
Wilton, Word Origins
Etymology of Medical Terms and Body Parts
The Phrontistery: International House of Logorrhea (and more)
YourDictionary Library (link library--word-geek treasure trove!)
Linguistic Atlas Project (Dictionary of American Regional English)
Bartleby.com's Library
Merriam-Webster Online
Signs, Symbols, and Sigils
Words That Should Be Banned
Language Log
Chiasmus (the art of turning words back on themselves)
The Power of Words (lessons on writing well)
Ask Mr. Language Person
Nuts and Bolts of Writing
Classic Errors (and help avoiding them)
The Anglo-Norman Hub
History On-Line, 13th century links
History On-Line, 14th century links
History On-Line, 19th century links
The Why Files
Popular Science archives (complete with advertisements)
wiseGEEK
Victorian Slang
1811 Dictionary of the Vulgar Tongue
Latin quotes and phrases
Wolf Proverbs
Wolves Of The World
Medical Dictionary
Rhetoric
Southwest School of Botanical Medicine (library of herb images and uses)
Project Gutenberg (free online library)
Conversion Calculator
Alternative Rules of Citation
How To Do (just about) Anything
Perpetual Calendar
Sunrise, Sunset: times by location
Historical Clothing
The Costumer's Manifesto
History World
In Ferro Veritas, exploring the martial art of incurable romantics
Regency information links Military Theory library of links
a list of names with reasonably good meanings
Behind The Name
GAMES
Etymologic!
Brain Food Puzzles
Oxford Word Challenge
Vulgar Grammar Video Ignore "loading" message. Video plays when arrow turns white for your cursor.
Your Dictionary gameroom
WORD OF THE DAY sites
Wordsmith.org
National SCRABBLE(r) Association
Ask Oxford
Cambridge Dictionaries Online
Dictionary.com
The New York Times
topical words at OneLook
(often impolite) words at Urban Dictionary
Worthless Word For the Day
Merriam-Webster Online
DATA RESCUE TOOLS and COMPUTER FORENSICS Penguin Sleuth
Some of these frequently go down, but if it looks interesting, keep checking. The link should either work or disappear pretty shortly.