Vocabulary/Spelling City is a free, interactive educational website that enables students of all ages and learning differences to practice their spelling and vocabulary words.
Knoword is a word game that focuses on teaching and advocating proper spelling habits. This is an activity that students could spend 5-10 minutes doing in a lesson or you could set it for homework. A downside is that currently there is no way to check if students have completed the activity.
Visuwords is an online graphic dictionary. It lets you look up words to find their meanings and associations with other words and concepts. It produces diagrams reminiscent of a neural net to help students learn how words associate.
Grammar Girl is a website that has podcasts on grammar. They are styled as short helpful tips. Hint: why not get students to create their own podcasts?
Grammarbook.com is a website contains free grammar tools such as rules, commonly confused words and grammar and punctuation videos.
My Vocabulary - is a free resource that helps students master vocabulary. There are free interactive puzzles focusing on using latin/greek roots, thematic puzzles on a variety of subject areas and word lists.