Is it appropriate to submit a previously presented workshop paper to a peer-reviewed journal? I have a paper that was presented at an NRC workshop and was not peer reviewed.  Moreover, although this work is accessible at an NRC website, it is clear from reading the literature that search engines do not discover the paper and no one is aware of its existence.

Is it ethical to submit this paper to a peer-reviewed journal for publication?
Different journals have different standards for what counts as prior publication.  For example, most computer science journals happily accept "extended journal versions" of existing papers that are intended to supersede the prior publication, as long as there is at least 30% new content and the relationship to the prior paper made explicit.  Some high-ranked biology journals, on the other hand, are so obsessed with "novelty" that they will consider a submission improper even if only an extended abstract has previously appeared.  Check the policy of the journal(s) that you are considering: either it will be listed clearly online, or the editorial staff should be able to give you a quick answer about their policies.