Today I have tested several monolingual dictionaries.
I tested them by writing 5 words in each dictionary and compared them afterwords.
The three dictionaries i think is the best:
1: Clue, it is a very neat dictionary that is easy to use. It is also a plus that you have to dowload it so that you can use it if you are ofline.
2: http://www.merriam-webster.com/dictionary/drought, this dictionary is one of the neatest online dictionary. It is a plus that it has list with other words that has your searchword in it. there is also some example sentences with the word you search for.
3: http://nhd.heinle.com/home.aspx, this is quite similar to the 2. place dictionary, but i am pulling it down because you need to click on the word you searched for to get it up.