Ever know how a word starts, but for the life of you can't spell it well enough to look it up? Try the Instant Word Search. This gem of a site starts popping words at you until it finally hits the one you're looking for.
Or are you looking for synonyms? Antonyms? Rhymes? Try The Big Huge Thesaurus. Okay, so the title is a bit redundant, it's still a fun site, particularly because it also generates blog post ideas and story/plot/loglines--a one-stop shopping site.
Of course, RhymeZone is still a popular site for finding rhymes and more including pictures and quotations.
Webliography is a terrific resource which has a list of terrific resources for mystery, romance, historical, and Christian writers.
And speaking of historicals, Our Time Lines helps the writer fill in the gaps and set the scene with a rundown of all the major events during the era in question. Easy to use, too.
But if you're just looking for some writing humor, you've got to see Inkygirl, if you haven't already. This site promises something for every writer's funny bone!