BBC learning English
A popular website not just for practising grammar, but other aspects of English.

British Council
A grammar practice site for all levels.

Help for English
A Czech website with clear and detailed explanations in Czech, plus lots of interactive exercises!

Perfect English Grammar
A website run by an Irish teacher of English which provides clear explanations in simple English with lots of examples and interactive exercises.

Cambridge English
Similar to BBC learning English, a website that provides an opportunity to practise various aspects of English with grammar being one of them.