Collocation preferences represent the commonly used expressions, idioms, and word pairings of a language. Because collocation preferences arise from consensus usage, rather than a set of well-defined rules, they must be learned on a case-by-case basis, making them particularly challenging for non-native speakers of a language. To assist non-native speakers with these parts of a language, we developed AwkChecker, the first end-user tool geared toward helping non-native speakers detect and correct collocation errors in their writing. As a user writes, AwkChecker automatically flags collocation errors and suggests replacement expressions that correspond more closely to consensus usage. These suggestions include example usage to help users choose the best candidate. We describe AwkChecker's interface, its novel methods for detecting collocation errors and suggesting alternatives, and an early study of its use by non-native English speakers at our institution. Collectively, these contributions advance the state of the art in writing aids for non-native speakers.