It took a bit of time and effort on the part of myself and Martin Sevior, but AbiWord now features a working grammar-checking plugin for English, based on an adapation I did on CMU's link grammar project. The plugin should be considered alpha-quality at the moment. We're contemplating building some sort of plugin loader compatible with OOo's lingucomponent, if they've managed to build a grammar checker yet (this page suggests that they haven't yet, but it's likely outdated).
As always, screenshots speak louder than words, so here's AbiWord 2.3 doing grammar-checking ala MSWord (i.e. ugly green squiggles):