Dictionaries!

The best english dictionaries (in my opinion...) have interesting free online versions:

1. Longman:
http://www.ldoceonline.com/dictionary/search_1

I love longman definitions. The American contemporary english dictionary is the one I have in printed version, but, the site is a bit complicated to use

2. Cambridge
http://dictionary.cambridge.org/

For me, this is the best site, it's simple and you can choose the dictionary according to your english level(advanced learner's or learners) or interest (idioms, american english, phrasal verbs). Another interesting thing: you can listen the pronouciation of the word with american or british accent!

3. Oxford
http://oxforddictionaries.com/

this site is very fast, and they have interesting activities on the section Puzzles and Games

Exercises

1. Look up these five words on Cambridge Learner's dictionary website and create sentences with then(don'copy the examples they give!):
-heaven (noun)
-shy (adjective)
-sell (verb)
-apologize (verb)
-give up (verb)

2. Do you know what an anagram is? Go to the section Puzzles and Games of Oxford dictionaries
and put this words to discover:
-kela
-recaami
-bciml

You can answer the exercises on te comments or write me an e-mail!