There are loads of websites which try to help you with your English grammar. A few of them are good, many more of them are, to be honest, absolute rubbish. I'll try to give you what I think are some of the better websites which focus on grammar. Don't forget, good newspaper and magazine sites are often a great source of well-written English.
Here are some of my favourite grammar sites:
What is a sentence? (RMIT Writing Lab)
BBC Ask about English
British Council
Grammar Movies from the BC
Grammar Quizzes
Edufind English grammar
University of Victoria Reference Tools
Viv Quarry - this isn't a visually attractive site, but the information is very reliable.
Grammar Bytes
Ego4U - it's not pretty, but it's useful
Free English - Some nice, simple grammar
University of Illinois - Grammar Handbook
Guide to Grammar and Writing - CCC Foundation
Here's a diagnostic grammar test - try the Intermediate test to make you feel good (hopefully..), then do the Advanced and see how you feel after that...
Edufind's Online English grammar is a useful reference for basic grammar information
A useful guide to some tricky grammar areas can be found at the Centre for Independent Language Learning
The BBC Learning English Language Bank is a useful place to look for information on grammar
1. State verbs
2. Adverbs of frequency
3. Articles
4. Comparatives
5. Conditionals
6. "
7. Uncountable nouns
8. Verb patterns
9. "
10. Structures with 'wish'
11. Linking words
12. Modal verbs
13. Gradable/extreme adjectives
14. Passive
15. Question word order
16. Relative pronouns
17. Verb patterns+prepositions
18. to have something done
19. Present Perfect (for/since)
20. get used to
21. so/such