Sometimes ago I asked a question and a very nice answer was provided. I feel that the answer was not sufficiently upvoted. Unfortunately I missed the deadline for starting a bounty for rewarding an already accepted answer. I know that the person who provided that answer doesn't need reputation but still I ask this question to know if something can be done in such situations.
(See the answer to "Can I award a bounty to an old answer?" in this FAQ list)
Rewarding a "nice answer [which was not] sufficiently upvoted" is one of the intended use-cases of bounties.
The benefit for the answerer is two-fold: he gets the additional rep from the bounty, and he gets upvotes from the extra-visibility given to the question.
Another course of action, for users who want to reward an exemplary answer but do not have enough reputation for a bounty, is to draw visitors to the page. For example, you can feature the question in your blog, tweet about it, and so on.