Free online ELF / ESL Grammar: Concordancers & Safaris
There are lots of grammar quizzes and exercises on the Internet.
You'll also find grammar explanations but they are often over simplified, don't take into account modern usage, and sometimes are just plain wrong.
Here are some other ways of investigating how English is really used:
British National Corpus 1980s-1993
The concordancer will give you examples of how is a word is used in context. Based on several large corpora of modern spoken and written British English. Gives complete sentences.
Take a five minute guided tour to understand how it works.
Concordancer based on Voice of America's special English Programs
Complete sentences given.
The examples are easier than the other concordancers.
Tim Johns Data-Driven Learning page
Concordance based exercises.
The Compleat Lexical Tutor
Université du Québec
A very original site with a lot of different ways of working on words. For some of them you have to down load a plug-in.