Oxford University Press has a great site for sentence structure and phrase building for a variety of skill levels.
Go for Grammar Gold has several exercises for beginning English learners. This is a great site for beginners!
Grammar Girl is a witty podcast put out by Mignon Fogerty. Who knew grammar could be so fun?
English Page provides quick tutorials and quizzes to help you understand modal verbs
Quick reviews of practically everything you need to know about grammar and writing.
Tutorials and quizzes of hundreds of grammar topics.
A grammar table of contents to help you look up practically any grammar topic.
Practice your grammar with more than a hundred grammar quizzes.
Get your daily dose of grammar with Daily Grammar.
Click on this Grammar Glossary for a definition of grammatical terms.