I'm going to disagree with the majority here, at least in part.
Obviously, a gift isn't required; all you really need to do is write your prof a nice, sincere thank you note. But IMO, if this person really went above and beyond the call of duty, there's nothing wrong with a small, token "thank you" gift you think she might enjoy. (Flowers or something edible are always nice, as they don't need to be kept around long.) The emphasis here is on small, as it should be obvious that you're just saying thank you and not trying to bribe or "pay off" the professor.
I would agree, though, that if this prof still has other letters to write for you or you're currently in her class, you wait till that's no longer the case ... otherwise, it looks like you're trying to suck up to get a better grade or recommendation.
And congrats on all your acceptances!