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?
This is a legal question which you should ask a lawyer to be certain. Given that SmugMug 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 where 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 would be claimed if there was no commercial gain for 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
One purpose of a model release is to warrant to a commercial agency that the subject of the photo was paid and consents to the photo being used commercially without limitation (or as specified in the wording of the document). If you are not selling the photo to an agency that requires a model release, then you don't need a model release, as it is not a legal requirement to have one in order just to publish a photo. If there is the possibility that at some point in the future you may sell the photo commercially, it is good to have a model release from the start (it's not practical to get the model to sign a release years later).
However, even if not strictly required, a model release can still have a benefit: it is a signed document by the subject in the photo indicating that they consent to a certain usage of the photo. Such a document may assist in your defence if the subject ever tries to claim that various other rights such as moral rights have been violated by the way in which you publish your photo. It is not an automatic win, it just may assist you. So while it may not be a legal requirement to have it, if your subject is willing, it can be a nice thing to have.
Obviously, people publish photos all the time that were taken with no permission and where the photographer got no model release. Photojournalism and street photography are two obvious examples, but other examples are, just "holiday snaps" - more and more people are publishing their personal photos in public spheres, and there is no need for model releases the majority of the time.
You would need to be concerned primarily about what other rights of your subjects you may be violating - was the photo taken in a private space, does the way you have published it make it seem like the person is endorsing something or doing something that may tarnish their reputation? Even with a model release, these can become problems for you, but having one might put you in a better position than not having one.
This doesn't have a certain answer (in the U.S.)—there's never been enough money involved to get far enough in court to get actual precedent set. Someone always blinks.
The closest we've gotten is the diCorcia case. https://en.wikipedia.org/wiki/Nussenzweig_v._DiCorcia but even that had the appeal decided on statute of limitations grounds, not the underlying facts, so no precedent.
Although the trial court's opinion (in diCorcia) that the 1st Amendment can trump right-of-publicity laws for artistic expression is an idea that should loom large in any other presentation of this issue.
No precedent means no chance of a cheap summary judgment/dismissal. If you piss off someone with the money to sustain a spite lawsuit winning is nearly as expensive as losing. Tiny risk, potentially huge consequences.
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.