List of websites teaching Tamil Grammar :
Tamil Virtual University grammar courses
Books on Tamil grammar listed at Viruba.com
Tamil Wikipedia grammar guide - Has relevant links, guides for understanding essential Tamil grammar.
http://ilakkanam.blogspot.com – A team blog in Tamil answering common questions / doubts on Tamil grammar.
அடிப்படைத் தமிழ் இலக்கணம், பொற்காலத் தமிழ் இலக்கணம் – Free e-books for download from Noolaham.net.
Tamil grammar review – Compilation of reviews, texts like TholKaappiyam, downloads, guides about Tamil grammar from TamilNation.org
மரத்தடி.com articles on Tamil grammar - Caters to advanced level students of Tamil grammar.
Tamil grammar through English – A simple beginner level presentation in English to understand basic Tamil grammar.
http://www.pudhucherry.com/pages/gram2.html andhttp://www.pudhucherry.com/pages/gram3.html teach you about சந்தி இலக்கணம். i.e, when to add க், ச், த், ப் in between two words when you write them together. Guess this is the number one confusion for any tamil writer 