This is a great blog if you want your students to practise their English with music!
Dictionaries:
The Macmillan dictionary is great for looking up words but it's also got a blog, a buzzword section and a crowdsourced dictionary!
The Oxford English Dictionary contains over 600,000 words but also has Word of the day and lots of helpful resources!
Grammar:
Here are some great links for students to practise their grammar:
http://www.perfect-english-grammar.com/grammar-exercises.html
http://www.esolcourses.com/content/exercises/grammar/english-grammar.html
Pronunciation:
To find out how to say a particular word, try HowJSay!
BBC Learning English have a great area to help improve pronunciation.
5-Minute English have a variety of pronunciation practice pages too.