I agree with Aliza that it depends a lot on why you're leaving, where you're going, and your level in the organization. Two weeks is still the standard. I'm in an organization where people have given less, and I think it's an insult unless they're leaving due to a true personal emergency.
I would never give less than two weeks, but I've given as many as six, which did allow for hiring and some overlap with my replacement. If you and your employer want that, longer notice is definitely better, especially in a higher-level role. However, if you're headed into another position, how long they're willing to wait for you to start is a factor too - since they
might want some overlap and training with you and the person you're replacing there.
The best deal is giving enough notice so that you can pad the time between your last day at the job you're leaving and your start day at a new job, since you may not get vacation for awhile.