Dictionaries TWL98/TWL06 usually for USA/Canada; SOWPODS for UK, Australia, Europe etc
Board Tiles: You can enter letters/words from the board and get words that can be combined with them.
Prefix/Suffix: Filter results by specifying a required beginning or ending of words
Word Counts Only: find number of possible words only
Score Empty Tiles: choose this if you want question mark letter to be scored
Low Resolution: click this, and answers will be unformatted and will load much faster