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If I'm watching an aircraft fly overhead and it becomes apparent to me that it is failing, what action can I take in response to this incident?
For now I will leave this question generic as to location, but if it becomes necessary to narrow it down I am interested in the US specifically.
Honestly, very little.
First, be clear as to whether you are actually see something failing. If you see flames, or a wing has broken off, then yes, it's a problem. However, if you see someone doing a slow spiral towards an empty field with the power idling, they're more likely to be practicing engine out procedures (something that's required for all US pilots to practice and demonstrate during their checkride for their certificate).
In the event that you actually see an impact, fire, off-airport landing and so on, call 911 and give them your location. They'll send the appropriate crews.
Otherwise, if the aircraft is still flying, there's a strong chance the pilot(s) are already talking to ATC about their problem, if it actually exists. There's nothing you really can or need to do in this case.
Quick Anecdote: I've been told the story of a local police department who gets phone calls virtually every time a citizen lands his small airplane in his private field. Fire/police/medics/etc are summoned, when absolutely nothing bad has happened. This is why you should make an attempt to verify that a problem actually exists before reporting it.
Brian Tusi's answer is correct in that there's probably very little (most likely nothing) that you can do right then and there to help that particular aircraft, even if your assessment is correct in that they actually are having serious problems.
However, there is one thing you can do, and which has proven to be helpful in real-world accidents and incidents: document as much as you can, as quickly as possible.
Pull out your phone, or camera, or whatever else you might have handy that takes pictures or video. Set it to action mode if you have the time, but don't spend a lot of time fiddling with the settings. Pointing it at the aircraft, start photographing or recording video. If you're recording video with sound, say something about your location, the current time and maybe the direction you're looking in, in such a way that it gets recorded. (Especially cell phones will usually record your location in the image metadata; I'm not sure about video, but either way, it likely doesn't hurt to help.) If you're using a still-pictures camera such as a DSLR without video capability, you probably want to set the focus to manual and infinity, and then just keep snapping as many pictures as you can for as long as you're able to see the aircraft. Focusing at infinity will almost certainly be the right setting for anything at any distance where you're likely to be able to survive observing an aircraft accident without injury to yourself, and setting the focus to manual will keep the autofocus from changing the focus distance at an inopportunate moment.
I would advice against posting the video or pictures to any particular online service, for both social and technical reasons. Considering the needs of accident investigators should this be an actual aviation emergency, such services often recompress any video or images uploaded, which can lose detail, as especially video compression is almost always a lossy process. It's probably better to keep it as pristine as you can for the time being. Keeping the video to yourself also reduces the risk of influencing others, which besides possible impact on victims, their relatives and others, could potentially make investigators' jobs more difficult in trying to piece together what actually happened.
Once the immediate situation is over, you can contact your national aircraft accident investigation board, whatever they might be called locally. (NTSB in the US, AAIB in the UK, etc.) Ask them if there's an investigation ongoing for an event around the time when you saw something odd; if there is, offer to send them what you have, and work with them on how to deliver it to them. It's possible that they will want to interview you as well, as a witness.
If there is a bona-fide emergency going on, then those pictures and the details you might be able to provide in an interview just might provide accident investigators with just the clues that they need to figure out what happened. If there's not, then all you lost was a few minutes stopping and recording what was going on, and maybe a few minutes spent on looking up their phone number and making a phone call; after all, with digital photography, you can always just delete the pictures or video later.
As a final note, if you come across what you have good reason to believe is aircraft debris on the ground, don't touch it in any way; contact the local police. It's likely that they will want to get that to an aircraft accident investigation team; your contacting them may help them do so more quickly.
In addition to the suggestions in the other comments, you might do what this guy did, because your scenario just happened a few days ago:
Pilot praises RAF Lakenheath plane spotter for saving his life
A plane spotter who alerted an airbase to a jet "malfunction" averted a potential disaster, its pilot said.
Ian Simpson called RAF Lakenheath after seeing flames and sparks come out of Maj Grant Thompson's F-15E Strike Eagle on take-off.
Photographer Mr Simpson regularly watches jets take off from the United States Air Force base in Suffolk.
The pilot said Mr Simpson's quick actions had "100%" saved his life, adding: "His initiative was awesome."
Obviously it helps if you know where you are and what you're looking at, and in this case it seems as though it was fairly clear that there was a significant problem.
