Write Right

Oxford Advanced Learner's Dictionary - good for non-native speakers, British English

Merriam-Webster Online Dictionary - general dictionary & thesaurus, US English

Burchfield - Fowler's Modern English Usage - British English

Follett & Wensberg - Modern American Usage: A Guide - US English

Purdue University Online Writing Lab - Good overview of grammar points & non-native English problem areas

FAQ at Ask Oxford - Commonly asked questions about English grammar