I was in a park yesterday snapping photos. Though focused on landscapes and nature shots, I happened to get a photo of a woman in the park and it turned out to be a nice photo. She was hundreds of feet away and I wasn't prepared with model releases or anything, so I didn't bother running after her. I'd like to post the photo on my SmugMug portfolio. I don't plan on selling the photo or using it in any other manner. Will posting it on an online portfolio get me into any kind of trouble? Am I better off just leaving the photo on my hard drive?
That us a legal question which you should ask a lawyer to be certain. Given that SumgMug is in the middle, you may want to check with their terms of service too. The particular legalities will depend on where you live and wheere the photos are hosted.
As a general rule, if you have to ask then you should get one. On the other hand, the likelihood that you get into trouble is proportional to your perceived ability to pay.
You may simply be asked to take down the photo and I do not think much damages claimed if there was no commercial gain of you or SmugMug. You definitely need a release if your page has ads or offers prints, regardless if it is you or your provider doing the printing.
In most of the world it's perfectly legal to use pictures you took of people in a public place with some simple limitations.
Unfortunately those limitations are completely different between countries and even between states in the US.
Generally, if the picture isn't used in an offensive way (or a way that can seem even remotely offensive to someone else) and doesn't imply the person in the photo endorses anything and the picture isn't used to make money you are OK - however - I'm not a lawyer, I don't know the law wherever you are (I don't even know where you are), this is not legal advice and even if it was you shouldn't take legal advice from strangers on the internet.
So, use your common sense, think how would you feel if you accidentally found a picture of you in the same situation and be respectful to other people.
Update: unless you have permission you can always get sued, see Clara Onager comment below
If she is identifiable in the photo, then yes, you need her permission (model release). If she is not identifiable and this is in public space, then no, you don't need one.
If she is identifiable (as in "can her own mom recognize her in the photo") or not will always be a subjective evaluation. If this cannot be determined without any doubt, then a model release will give you a safe card.
Will you get in trouble if you don't use one, and she is recognizable? That will depend on the woman. You see where this is going.