I know that Word has a basic grammar checker, and that some can be downloaded for open office. That said, you’d need to export for these to work. I know from writing for school (and even the short story class I took way back), that grammar checkers can sometimes not be all that effective. They can’t, for instance, tell you if a sentence just sounds wrong when said out loud, even if it may technically not have any grammatical errors. I stopped using grammar checkers a LONG time ago, and actually find it more convenient to read it over a few times myself, and then pass it on to a friend or something who is willing to take one more pass at it.
I think the issue is that, to me anyways, grammar is a pretty complex thing (especially English), and it would be difficult for a computer algorithm to accurately detect the entire context of a paragraph. For example, writing for science at least, you want a separate topic for each paragraph. In order to tell if your paragraphing is correct, any program would need to know that every sentence in your paragraph is about a certain topic. Unless you have repetitive writing, I don’t know if this could be done by a computer, and repeating the subject in every sentence is a writing problem in itself. A computer also can’t tell you if your arguments, or flow, makes sense, so the project would still need to be proofread.
It would be nice if a computer could do all these things. Then I could spend say… 2 months writing my paper, click a button and have it be nearly publishable, netting me a nice 80-90 in my class, rather than spending 1 month writing, another month editing, and wishing I still had 2 more months to edit. However, if you look at any of the authors who are big on their community involvement, editing is one of the most intensive parts of any project (even in science, sure, your stats are phenomenal, but if your paper isn’t persuasive, you’ll have a harder time convincing people that your study matters).
As for alternatives, I ask friends (or acquaintances who are mildly interested in the topic) a LOT. I’m not sure what kind of project you need help with, but I find that to be the most cost effective way. Also, I don’t know if you’re near any universities, but I’m ALWAYS seeing flyers up for students looking to make a few extra bucks by proofreading. Not sure what they’d charge (I always just bribe my friends with cookies…) but it could be something worth looking into.