Some questions continue to percolate even though they have an accepted answer. Is it generally O.K. to enter an on topic response afterwards?
Assuming, of course, that you've something useful to add. "Accepted" just means the person who asked the question found the answer helpful - it doesn't mean that answer is necessarily the last word on the topic. If you think you can provide a better answer, or even just answer from a different perspective, then by all means - post it!
If you can edit your information into an existing answer (i.e. it's supplementary rather than contradictory) that's better than a new answer.
Otherwise, new answers to already-answered questions are a normal part of SE - though in my experience on other sites, they are usually not great quality (most often restatements of an existing answer, and usually not as insightful) so I find I tend to downvote them anyway.
As Shog9 and Joe state (and demonstrate): Yes, there's nothing wrong with adding a useful answer to a question that already has an accepted answer - indeed I remember there being a discussion about how we could tactfully suggest that new users don't accept answers straight away on SO, because seeing that a question already had an accepted answer was perceived as discouraging others from answering.
It's also useful when technology or techniques have moved on - especially on programming sites where questions might be about Beta software, but even here, if the LEGO Group were to release a new brick that makes a particular form of SNOT building easier, then by all means post more answers about it.
Generally though, usually it's just people saying "Thanks that helped me too" because they can post an answer with no rep, but can't add a comment.