University English is primarily intended for international students who want to improve their English language skills, but the site is full of resources for English language students at an intermediate level and above -reading, listening, writing and English language issues are dealt with in a systematic way.
It is on University English that I first heard of the term comma splice, that is, the incorrect use of a comma in between two sentences, instead of using a conjunction to link the two sentences together, or simply separate the two sentences with a stop.
It is nearly half past five, we cannot reach town before dark. (comma splice)
It is nearly half past five, so we cannot reach town before dark. (use of a connector to link the two sentences)
Funnily enough, right after reading about the comma splice on University English, I just noticed how often this problem presents itself in students' writing of all levels.
Patrick McMahon suggested taking a look at an OWL’s powerpoint on the use of commas, conquering the comma. Pay special attention to slide 24 and following, where the problem of the comma splice is explained.
And make a point of dropping by University English on occasion. You won't regret it.