I have often come upon questions where the asker never bothered accepting what is obviously a correct answer. "Obviously" can mean (aside from my own judgment):
The answer is a clear leader in up-votes (again, combined with my judgment that it is a correct one)
The answer is the only one (and again appears correct)
The answer (or Q) is commented on by OP indicating it helped.
If the user is still active on SO, what helps sometimes is to post a comment on the question mentioning you can/should accept the answer. For my own answers, this technique worked ~30-50% of the time.
However, if the user is a zombie (showed up >1 month ago, asked one or few answers, and was not on the site for >1 month since), this would obviously not work.
To ameliorate this situation, I would propose allowing users whose rep > threshold to "accept" answers from other peoples questions. This would be limited to:
questions from zombie users as defined above (e.g. total active time on site <= 30% of time since last visit, or however you want to define).
questions from non-zombie users with very low accept rates (this may require higher threshold).
In either case,
neither the OP nor "guest acceptor" gains 2 points for accepting.
the OP should have the right to choose a different accepted answer at any time, and get 2 points rep when that happens.
If you accept your own answer, you obviously don't get 15rep for acceptance.
I think if implemented to the above specs, it should provide enough of a barrier to misuse while serving the goal of the site (allow arriving at the best possible answer).
P.S. This was obviously triggered by some of my own answers but once I realized the situation I observed it numerously with questions I was merely browsing.