Dictionaries:

You know it already: Oxford Advanced Learner's Dictionary - try the online dictionary

Surf to the Cambridge International Dictionary of English which is targeted at learners of English. The definitions are clear and simple. You can search for words in the dictionary of idioms and for phrasal verbs, too.

Visit Merriam-Webster, which is not only an important dictionary with a thesaurus option, it also offers a selection of extremely difficult word games, puzzles, and definition dilemmas.

You might give the Wordsmyth English Dictionary-Thesaurus a try. It is easy to handle and offers simple definitions.

The very best German-English online dictionary at the moment: www.dict.cc 

Of course, leo is still online as well and quite useful: LEO.  

Grammar

If you need an easy online-grammar, test this one, perhaps it can help you.

Various web-sites to work on your grammar: Hamburg-list

Encyclopedia:

Amazing: Wikipedia with roughly 1,400,000 articles at the moment