Here is a list of online dictionaries which I tend to search around for words:
Macmilan Dictionary - Has a nicely done thesaurus based on specific word definitions
Onelook - This is good for reverse searches or searching with regular expressions
Worthless word of the day - A great place to find old or obscure words since it is a huge list with short definitions
The Phrase Finder - This is a great site because dictionaries are terrible at defining phrases
Using English - Another good idiom/ phrase site
The Eggcorn database - A site devoted to collecting the kind of unusual English spellings that have come to be called eggcorns. Sayings such as "curve your appetite" instead of "curb your appetite"
Tiscali - They have a list of difficult words
Double Tongued - Slang dictionary
Urban Dictionary - Most popular slang dictionary
Word Spy - New words with references to uses