dictionary and thesaurus

Best offine freeware dictionary I know of is TheSage.

If you don't know how to spell leviathon, (leviathan), no problem. If you get close, TheSage will usually pop up a list of words it "thinks" you ment. It also has a wildcard feature. Say that you think leviathon is wrong, but you know a vowel goes where the letter o is. Enter leviath#n, and use the wilcard feature.

If you want to help the kids to learn spelling as well as word meanings, you could easily make a spelling game, (first one to misspell loses, etc.).
http://www.sequencepublishing.com/thesage.html

A very good freeware "on the fly" spellchecker is TinySpell. It is useful to check spelling as you type in situations where you might not have a spellchecker, ( notepad, web based email, etc..) Tinyspell does not correct spelling, it simply alerts you when a word is misspelled. It can be configured to autostart with windows, or you could just crank it up when needed.
http://www.tinyspell.m6.net/

A good online dictionary is Wiktionary
http://en.wiktionary.org/wiki/Main_Page

another is Merriam-Webster online dictionary and thesaurus
http://www.m-w.com/