Lot of questions have appeared with horrible spelling and grammar mistakes on this site.
- Just downvote them? or
- Comment requesting the OP to modify the question? or
- Simply edit the question
It's really a judgement call depending on the situation. In general, it can be said that such questions fall into two separate categories:
There's really nothing wrong with those posts in the first category: once you get through the veneer of mangled English they're still high-quality posts, and we want high-quality posts.
The second category, however, is another story. Mangled English or not, they're still low-quality posts and we just don't want to attract those.
Distinguishing one from the other is difficult (and often highly subjective). But in general, regardless of how fluent one is in English, someone who actually puts in the effort to make themselves clear will actually succeed, or be very open to constructive criticism suggesting how to improve their post (it's often much easier for a non-native English speaker to read English and take advice than it is for him to write it). Those in the second category, on the other hand, won't.
If the underlying core of the post is valuable enough that you want to actually bring it more attention, go right ahead and just edit it; this is one of the foundations of the Stack Exchange model for "making the Internet a better place." But for myself, I wouldn't bother editing to improve those posts in the second category, as that's effectively just rewarding laziness and sloppiness. Those should just be downvoted or, depending on the severity, outright deleted.
3. Simply edit the question
If you think you can fix those spelling mistakes and grammar without changing the meaning of the question itself, please edit the question. It is important that questions are well written and should attract a decent answer.
We should not down vote a question based on poor grammar and spelling alone. By that we are really discouraging new questions. If it is worth the question, we should edit it and fix if we think we can!
If a user has poor grammar/spelling, he simply can't be bothered to fix it himself, after all he has poor spelling/grammar :)